Unfortunately the "exotic" mercenary brigades are a little awkward right now. In the long run I'm looking into a new mechanic to handle the hiring and recruitment of "off-the-map" mercenaries so that they can be used in the sandbox as well as the campaign. But unfortunately that probably won't make it into the game in the immediate future. In the meantime they're probably considered Roman and can therefore only be assigned to other Roman cities.
I suppose it's OK as is, what made me worried was that they seemed not to be getting reinforcements, but that may have been just result of the only rome city being dried of recruits. Later I noticed they were at full numbers again, so no problem. Although not sure if I looked at details of that unit. Anyway, it's chapter 1 only, so... no problem. And thanks for reply.