I have two orange (red?) left (one was a jumper, and missed the tank). From what I've heard, both the purple and red firefish are very hardy. I have heard that the purple don't tend to tolerate eachother very well, though.
There were three red in my 120g that got along just fine. I'd only see one dart at the other once every blue moon. Had them since around March of this year, I guess, and I can probably count on one hand the amount of times I've seen them dart at eachother.