Merc's only really work in RL because they have so many countries and corporations to provide their services. Though Blackwater International recently changed its name to Xe, so that they could continue their contracts in Iraq and so the US didn't have to hire a bunch of new guns or increase troop levels.
The thing that also needs to be pointed out is that when the Bretonia/Kusari War is over and the Rhineland/Liberty Conflict is end is that this Merc company could either assist police forces with pirates, or escort trade companies. Typically though they would want to avoid illegal services since house factions have more power rp wise and thus during peace time you are an escort service for traders.
The idea is sound but Freelancer is to small for it. The BAF would rather recruit new pilots than hire a trained force they had to pay until the end of a contract. After the war maybe like a rl year maybe the Kusari may want to hire your Mercs. The purpose of having this faction is to bolster the size of a house military in a time of need.
Another idea is that this Merc force would be dormant when they dont have a war to fight. They would only come together when a contract was accepted. I had thought of this for a while but these things dont last long.