In my (limited) experience: Generals (family members) will join your faction if successfully bribed, no matter what faction. If the army is not the same culture as you, the troops will disband.
For example: As Julii, when I bribe any Brutii troops they will all become Julii. When I bribed a Gaul army commanded by a general, I got the general, but his troops disbanded. If I offer a bribe to any non-Roman or Rebel army they will always disband.
And, yes, catch them outside of a city. Bribing a settlement is much more dificult.
Uh, huh?