Po choosing Zhen instead of all the other well experienced Candidates as his successor as Dragon Warrior fits very well on why he was chosen, even if Po doesn't realize it. Po was never trying to be the Dragon Warrior, he was merely a fan of Martial Arts and wishes he could learn them too, heck, he was merely trying to see who was going to be chosen, and was humbly chosen by Oogway in the end. Oogway never wanted Tai Lung, nor the Furious Five, to have the title since they only ever saw the Dragon Warrior status as a title, to empower their own self image, something that went against martial arts in the first place, much less what being the Dragon Warrior meant. Shifu running the test to see who Po would pick as his successor went against Oogway's choice because, much like Tai Lung and the Five, the candidates never actually needed it as they were merely after the title and are capable on their own. Po was eager to learn Kung Fu so much that he took the brunt of Shifu's abuse and lack of Shifu trying to teach just to have the honor of learning, not because he was The Chosen One, but because he wanted a chance to improve his life, which at that point felt meaningless to him. Zhen never sought the title either, but by the end of film, she showed her potential and more importantly, willingness to change is what got Po's attention to humbly choose her as his successor. To be the Dragon Warrior, one doesn't have to be a master, one must be willing to improve.
The five being more than accepting of Zhen as the successor, at least more than Shifu, is because they know first hand that Po was abysmal at first, but with time, effort and encouragement, he became a master worthy of everyone's praise. Like to be the Dragon Warrior, the Five all improved their views and are openly willing to train Zhen as an equal.
The scene where Po avoids choosing the new Dragon Warrior by choosing himself is mainly meant to be a reference to the first film, but it's also foreshadowing why it's Po, and not Shifu, who's ultimately best suited for the role of Spiritual Leader of the valley. By trying to force Po to make the decision, Shifu was falling into the same trap that he was trying to help Po avoid: trying to force things to go in a way that's convenient or comfortable for you, rather than accepting change when it comes. Shifu is just experiencing the other side of the problem: attempting for force a change that's not ready to happen yet rather than waiting for the right time. He's right that Po will eventually need to accept not being the Dragon Warrior any longer, but Po ultimately does make the right choice; as of that moment he is still the Dragon Warrior, and the time hasn't come for him to vacate that role yet. He didn't know it at the time, but it isn't until after he meets Zhen and mentors her the same way that Oogway mentored him that Po is truly ready to step into Oogway's shoes and become a spiritual leader rather than a warrior.
The Chameleon being turned down as a kung-fu master for being small seems odd considering the likes of Shifu and Mantis, however if one goes back to the first film, they had their own prejudices against Po, who they deemed too big and clumsy to fit as one (complete with a Hypocrisy Nod on Mantis' part). Shifu similarly tried to inorganically teach Tigress his fighting style at first rather than let her define her own style. It actually seems a recurrent trend for kung-fu masters in this universe to be rather close-minded about their styles and approaches, just the Jade Palace warriors learned better due to their experiences with Po. It furthers the Chameleon as a Shadow Archetype to Po as she suffered similar discrimination in life, but chose to stew and spite those who judged her rather than prove them wrong and enlighten them.
One may also note that Mantis was initially turned away by Shifu along with the other Five members, not even giving them the time of day over the warriors he had intended to fight with (who were essentially clones of himself). It was only after Mantis and the others proved themselves by helping Tigress defeat Boar that Shifu came around.
The villains all being willing to show respect for Po has extra validity when you consider that his battle with the Chameleon was not just an act of him saving their spirits, but a display of trust and compassion to Zhen, becoming the Big Brother Mentor to counter the Chameleon's Evil Mentor. Tai Lung, Shen and Kai all have the mutual hardship of a broken parental/brotherly relationship. Seeing Po counteract one for Zhen and prove she could be redeemed may have touched a nerve for them.
Why didn't The Chamaleon tried to summon Oogway and steal his powers, well she likely knew that she stood no chance against one of the greatest Kung Fu masters of all time.
Additionally, given her "rules of the street", the Chameleon ran on Virtue Is Weakness. Even if she knew of him, Oogway being a gentle and sagely soul who only took action when he felt he had to likely left a soured perception to her. Indeed most of the warriors she chooses are bad guys.
The Chameleon lacks the same Color Motif that the other three villains had (blue for Tai Lung, red for Shen, green for Kai). This emphasises that she is not overpowering the scene through her own identity but instead through that of others, much like a real chameleon changes colour to blend in. The lair where she confronts Po is lit with a blend of the colours of all three previous villains.
While the TV shows have been stated not to be canon, however this is one of the few occassions where the films fit with the continuity of the shows, due to Po's comments that he broke the Urn of Whispering Warriors twice, but when it was broken in the third movie, it was Li who broke it, not Po, but if one takes Kung Fu Panda: Legends of Awesomeness into consideration, in Legends of Awesomeness Po accidentally broke it once when he bumped into a pig who was carrying it, so technically speaking Po did break it twice.
The Chameleon is the first main villain in the movies whose background isn't shown in any flashbacks even when she tells Po about her Freudian Excuse. With no tangible evidence to back up her words to the viewer, this serves to deny her any sympathy or even make it possible this well-established manipulative sociopath is lying. Even Kai, who wasn't provided with a tragic backstory like Tai Lung and Shen were, is shown in flashbacks to have once cared for his former brother-in-arms Oogway until the latter "betrayed" him. This, combined with the three previous villains showing Po respect upon the Chameleon's defeat, cements the latter as the irredeemable exception.
When Po offers the Chameleon a chance of redemption, she rejects it by shape-shifting into Shen and kicking Po across the hall. Notice how it's Shen she turns into. In the second film, Po offered Shen a chance of redemption, he rejected it. Kinda symbolic there.
And the real Shen witnessing this from his cage likely touches a nerve as he's more or less reliving his fatal decision as a spectator, which likely contributes to him peacefully returning to the Spirit Realm instead of trying to antagonize Po anymore.
Despite the film bringing back all three of the franchise's previous villains, the trio are never shown interacting with each other. While they have a common ground in their defeats at Po's hands, their differing standards and viewpoints - and even their implied past encounters in the Spirit Realm - would make any kind of camraderie between them impossible.
Tai Lung dedicated his entire life to becoming the greatest Kung Fu master, while Shen dedicated his life's work to creating weapons that would make martial arts obselete. Coupled with Shen's genocide against the pandas - which would likely touch a nerve with Tai Lung, being an orphan himself - the two would never get along.
In Kung Fu Panda 3, a brief glimpse at Kai's belt reveals Tai Lung was one of the Masters that Kai defeated and captured, and with this film confirming that Shen also resides in the Spirit Realm, it's likely he was captured by Kai as well. So it makes total sense that they'd both keep their distance from him.
In the previous trilogy, each villain represents the past of one of the heroes. Tai Lung was Shifu's former student, Shen separated Po from his family for decades, and Kai was Oogway's former brother-in-arms. While the Chameleon has no prior connection to any of these characters, she is the Evil Mentor of Zhen, and through defeating her Zhen ultimately becomes a hero.
As successor for the Dragon Warrior, Zhen has several parallels to Po's character arc:
Both were left without their natural parents and adopted by someone with very different ethics from their own.
Zhen, like Po, is an outcast, and is only allowed to affiliate with the Jade Palace masters out of neccessity. Her using being locked inside a prison as a means to strike her bargain also bears superficial resemblance to Po's ascension starting with him locked outside the Palace.
Similar to Po in the first film, the film gradually hints to her unhappy upbringing and insecurities as she confides in other characters, though we are only given the full story in a very blunt manner in the middle of the movie, Po when he calls out Shifu upon Tai Lung's escape, and Zhen when she betrays Po to the Chameleon. Both spill their inner demons by turning their back on their teacher.
Both end up selected as Dragon Warrior by means of Trickster Mentor tutoring. Oogway ascends to leave Po to deal with Tai Lung, while Po pretends to be captured to let Zhen face the Chameleon herself.
Additionally, Zhen had no ambition to become the Dragon Warrior, much like Po, being unknowingly chosen among a slew of more ambitious warriors who yearned for the title.
Po is arrested for traveling with Zhen. They later flee the Juniper City police, resulting in him getting his own wanted poster. Saving the day didn't undo Po's criminal status.
Granted only the authorities would care, the citizens would probably think highly of Po by association with Zhen.
The bunny triplets are using a baboon's tail as a paintbrush in one scene. Just one thing - the paint is blood-red. Knowing how vicious these bunnies are (although this is Played for Laughs), can we really be sure it's paint?
