A commander is you
I took the time to sort the factions into the respective categories, yet I'm not sure if I did it correctly. My results are as follows:
Romans - Generalist early on, later Brute Force
Pre-Marian Reform: Mix of light and heavy infantry, archers, skirmishers and light cavalry. Post-Marian Reform: Heavily armed and armored Legionary infantry, huge array of siege weaponry, mediocre ranged units, mediocre cavalry. Can feel like a Spammer when fighting them, since all three Houses will simultaneously wage war against you.
The Greek Cities - Brute force/ Specialist
Almost no ranged units, below-average cavalry, but increasingly heavy hoplites, including the super-tough Spartans.
Macedon - Brute force/Elitist
Some of the best hoplite/pikemen infantry, companion cavalry, not much else of notice.
Egypt - Elitist
Good all around rooster, with heavy phalanx units, archers and the ever fearsome chariots.
The Seleucid Empire - Generalist
Good pike-phalanx units, good cavalry, including cataphracts and war elephants.
Parthia - Ranged/Guerilla
Appalingly bad infantry, yet has one of the best cavalries in the game, with cataphracts, war elephants and horse archers.
Pontus - Specialist/Brute Force
Mediocre Infantry, but can recruit cataphracts and chariots.
Armenia - Generalist
Average all around rooster, with better cavalry than infantry.
The Gauls - Spammer early on, Specialist later
Lots of mediocre warband infantry, but gains access to Chosen Swordsmen and Forrester Warbands - the perhaps best archers in the game - later.
The Germans - Spammer/Guerilla early on, Specialist/Brute force later
Throw huge hordes of Spear Warband at you early in the game, aided by Screaching Women and Night Raiders, but later gains access to Gothic Cavalry, Chosen Axemen, Chosen Archers and Berserkers, who hit with all the force of an elephant.
The Britons - Specialist
Mediocre Infantry, no normal cavalry, but fearsome chariots.
Spain - Guerilla
Excellent skirmishers and light infantry and cavalry, but alomst nothing that can hold a battle line for more than a few moments.
Dacia - Generalist
Good all around, with a few stand outs, like Falxmen and Chosen archers.
Thracia - Brute force
Hoplites and phalangites combined with more barbarian shock infantry, not much in term of ranged units and cavalry.
Scythia - Ranger/Specialist
Focused on (ranged) cavalry to the point that their most basic unit isn't a warband like with other "barbarian" factions, but Horse Archers. Able to destroy pretty much anything on the open field, but rather ill-equipped for sieges.
Carthage - Elitist
Good Infantry with the Sacred Band as the top elite, but their true trump card are the elephants.
Numidia - Guerilla
Mediocre infantry, but lots of good skirmishers, both on foot and mounted.
In the Remastered Version, do Barbarian units, whose General is Bribed into joining another Barbarian faction, follow their general, if the enemy can produce those units?
The reason I ask this is, I used to play this game a lot back in the day, and since I liked to play for my own personal amusement, I liked using the More Money and Instant Building cheats, which let me just focus on what I really loved - The Fighting!
Now, one thing I liked doing was just sending my diplomates out and Bribe enemy generals into joining my side. Now, for the most part, if you were a Roman, Greek, Carthaginian, Eastern, or Egyptian, if you encountered a general, or enemy force, that was of the same civilization as yourself, and you bribed them into joining you, they might bring along some, if not all, of their force — It was real fun being a Roman during the Civil War. After all, get declared Outlaw, and 10+ full stacks of enemy armies march upon your cities, which may or may not be fully upgraded, especially the walls, and your local forces might not be fully upgraded either, or have a lot of combat experience. But, if you got a bunch of money, and some ambassadors.... well, that's when you make their armies yours.
The exception to this is if you were a Barbarian. Now, I get some of it — the different Barbarians had different units, but the different Greeks had different units, same with Eastern, Carthaginian, and even the Romans had unique gladiators (Only 1 Egyptian-type faction, but Egyptian Rebels popped up and could be completely recruited by them). The Barbarian factions — forget the Peasants, the Warband, the basic Cavalry — none would join the other faction, not even the Barbarian Rebels, even if said faction could recruit that unit. (Heck, while we're on the topic of Rebels, forget about playing as a Barbarian-centric Rebel, as those areas just wouldn't recruit forces if the culture shifted to a Barbarian one — you could get Greeks, you could get Carthaginian, Eastern, Egyptian, and Roman ones were also a pain to recruit, but at least they were Greek-type forces, unless you got lucky, and you could Bribe them into joining your side and thus you'd get some of the Pre-Marian based forces - and Gladiator rebellions could be fun.)
So, back to the Question - if say, playing as a Gaul with a boatload of money, and you encountered a Briton with say Peasants and Warband units, could you get not just the General, but some of those units to switch sides, and not just disband?