Well, if you want to experiment with "mercenaries", but be sure things don't get out of hand, you could do a double restriction.
1) Only one ship of a three ship fleet can be pirate
2) You may only have such a pirate mercenary for X matches in one cycle. (Say, max of two matches in one cycle?)
You'd stay pretty close to rcacial purity this way, but you'd still have an extra tool to reach for in a matchup against a race that your race didn't face well.
-S'Cipio