Yeah that sounds really cool and I want to make a special part of my army only foreign, but I always and up mixing my armies because of unforseen opportunities and such. Also, role-play wise, some factions are better suited to use mercs, like the privateers i'm playing atm. They don't care whose's fighting for them, as long as they conquer and loot some sweet gold
. And with other factions, like the Romuli, I like to use less mercs because they are more based on their own troops, I don't see them letting in non Romuli foreigners into their armies (which they would regard as "subhumans" of some sort).