It is a problem with all 'barbarian' armies. Essentially they can fight Romans, or each other. Some of the lucky ones get to be Carthaginian or Successor mercenaries or Roman auxiliaries. They still fight Romans, but they have pals with big pointy sticks, elephants etc.
I just have a couple of boxfuls of generic 'barbarians' for my warband requirements, and they've done the whole thing from being mates with Hannibal, to being hunted in dank Germanic forests, charging across the Sambre and riding around in Chariots rather hopelessly in Brittania.
Cheers
Martin