Awesome, that all makes sense, though none of it solves my particular issue (making friendly and hostile NPCs fight each other). :p
I'm having the opposite problem. I wanted to make a mission where the Terrans and the Remans were forming an alliance and they just beat the crap out of each other. Was wondering if there was a way to make them play nice.
Use a friendly group and change the individual costumes to Remans. They'll play nice then.