Word of God: Darwin Candidate has made posts explaining his reasoning for each of Team RWBY's starters, as well as "runner up" decisions and why he didn't use them. In general, choices were made based on Pokémon rarity, thematic connection to each character, whether or not the Pokémon would be considered a "pet" instead of an equal partner (or would overlap roles with Zwei the corgi), whether or not the choice would be considered cliché, and whether or not the Pokémon would be fun to write. Their second Pokémon all share a single post as they debuted together and the explanations given are simpler. They were all chosen from species found in Alola, where Team RWBY was staying at the time.
Ruby:
Zubat are common, bats fit Ruby's gothic fashion sense, Crobat are Fragile Speedsters like Ruby, and a friendship-based evolution means it can quickly evolve to Crobat without spending much time as a Golbat(or none, as it turned out!). Scyther was considered, but dismissed because its rarity, strength, and matching color palette (once evolved) made Ruby acquiring one seem Sue-ish. Eevee was considered due to Author Appeal and Rule of Cute, but Darwin felt neither it nor its evolutions were a good fit thematically, as well as the pet issue.
Rowlet was chosen as her second Pokémon to give her a fellow sniper.
Weiss:
Ralts are empaths that can see past Weiss' icy exterior, Gardevoir and Weiss share a "princess" aesthetic, the Gardevoir line's characteristic devotion to their trainers would provide Weiss with much needed emotional support, taking care of the mostly defenseless Ralts provides opportunity for character development, and, most importantly... Ralts are really, really cute. Glaceon and Alolan Vulpix were considered, but dismissed because they would be rare enough in Kanto that Weiss getting one would feel contrived, and would also run into the pet issue. Gallade was also considered as the possible final form for Ralts, but was dismissed due to the whole knight-theme surrounding it. Instead of a Princess and Knight theme, it was more of them being equals.
Piplup technically cannot be found in Alola in the games, but as Prinplup can be found via Island Scan, the author reasoned that a community of the line must exist to let multiple Prinplup be found. Empoleon shares both a 'cold' and a 'royalty' theme with Weiss, and one could use its shieldlike wings to shelter her and Whisper before all three attack.
Yang:
Torchic evolves into a Fire/Fighting type, and the author reasoned that of all the Fire/Fighting starters, Blaziken best fit Yang's Lightning Bruiser fighting style. Emboar was dismissed because it's a Mighty Glacier, and Infernape because it and a certain monkey-tailed Faunus share the same inspiration and would make a better pair. Several dragons were considered because of Yang's Meaningful Name, but the author decided they just liked Torchic better.
Jangmo-o is a dragon that punches, and Hakamo-o has scales that look like Ember Celica.
Blake:
Gastly's illusions and nature as a ghost-type compliment the ninja-like Blake, they have a third stage evolution like the other starters chosen, and the author thought pairing the normally quiet, reclusive Blake with a Pokémon known to be an outgoing prankster would be funny. Because Blake is afraid of dogs, and Zwei is already used for that joke, any and all canine Pokémon were out. Feline Pokémon were too obvious and were also out. Zorua was considered and dismissed because its Illusion ability could be too OP. Misdreavus was considered, mainly because Mismagius would have been the witch to Blake's black cat, but Darwin decided he could get more mileage out of Gastly.
Litten is a cat, like Blake, and Incineroar being a Large Ham would also contrast nicely with Blake's personality.
Darwin Candudate: I like the idea of quiet, stealthy, reticent Blake having Beetlejuice standing at one shoulder, and the incendiary feline equivalent of Macho Man Randy Savage at the other.
as they debuted together and the explanations given are simpler. They were all chosen from species found in Alola, where Team RWBY was staying at the time.