As has already been stated, there is no way to get around the 'recruitable merc' system. When looking at carthage in particular, you have a situation where the majority of the african, iberian, and gallic mercs were "monopolized" by Carthaginian generals, such as Hannibal, as has already been said in this thread. To center on the african and gallic forces specifically, the fact is that the most of the units represented in the game never fought for Rome once their respective lands were conquered. Gaesatae are not a "traditional" merc because it is accurate that they not be allowed to fight for Rome. The same with Elite African infantry, Iberian heavy cavalry, and other gallic mercs. This situation is unique, and is refected as so in the game. And we must remember that this is all non-negotable, as the game engine is simply not complex enough to handle any other type of merc system.