Thanks ya'll! They seem to be getting along pretty well. Oddly I thought the big guys would be ganging up on the 5'', but it is the opposite. The little guys are gettin' really sassy and are nipping the 4 larger rbp. Getting them together also seemed to make them less skittish. When I am in front of the tank now, they don't dart and hide, they are just doin' there thang'
I think jp said something about the number of piranhas correlating to the level of aggression/shyness......