sometimes I will put 12 to 15 in a pen for a week or so. Never have had any issues unless I throw in a hen. I also have the pen away and out of sight from the girls.
I recently put 2 teenage roos in an empty broody pen. It's working out so well! Those little guys were driving me nuts, and the hens hated them. Now they aren't bothering anybody. And they get to live!
I have a duccle roo and OEGB roo that are together and they get along just fine. And i also have 4 other roosters together and they all get along fine.