I've hunted there for months, and by experience, any Pirat kind can spawn at the same place but always the same amount. I mean one time its 3 Bombardier, 2 Cutthroat and 1 Mate, next can be 3 Mate, 2 Bombardier and 3 Cutthroat. But i can't say it's random because it has a balanced xp and a balanced amount of each type.