This is a fantastic idea - I've studied quite a bit of early-modern European military history, and a great deal of battles (and, indeed, wars) were fought with mercenary armies. King Christian IV of Denmark invaded the Holy Roman Empire using armies mostly comprised of mercenaries. What's interesting is that because the concept of nationality was embryonic, a mercenary fighting against armies of his home country wasn't even considered a traitor.
This led to certain nations becoming 'specialised'. For example, if you wanted the best, you'd hire English longbowmen, or Genoese crossbowmen, or Polish cavalry, etc.
This has indeed been a feature throughout history; the Roman empire, towards its end, was defended mainly by bands of barbarian mercenaries.
Such a concept would allow you to focus on being a purely mercantile civilisation (venice?) without worrying that the first sword-weilding meathead to come along will totally wipe out your empire.
I did actually think of the barbarians you could bribe in Civ II as being mercenaries. Thus, when I was playing a mercentile civilisation and a massive barbarian invasion of 16 legions occurred, it was actually a cue to recruit them and kick the stuffing out of my nearest neighbour!