Try it as Rome, the only time you can do it is those Italian rebel armies have the Mercenary Hastati, Velites, or Town Watch. I don't know how the Gladiator ones work. Greek style units are more widespread. I don't think barbarians can get the Merc Warband, because the icon is different. I have heard, I think on this forum somewhere, that Britain and Gaul cannot bribe each other's Warband to their side because the icon is different. Greece does not have that problem, the Militia Hoplites icon is the same, as is the Hoplite one (I believe). Probably same for Archers and Peltasts, I would have to look when I am in the game again.