My boys fight to establish a pecking order then they cuddle up together afterwards. I left one of my males alone for a while because I didn't have any males his size and I was worried about him picking on the smaller males so he got really depressed. Now I make sure he has at least one other rat with him, be that a male or female. He's much happier now.

So you can keep males together. They might have a little squabble but normally they settle down after that.