Normally rats do quite well together, whether they are all males or all females (never put a female in with multiple males however, that can cause major issues). One thing I always do whenever I introduce a new rat into an established group is give the incoming rat a good wash under some warm running water and a bit of baby shampoo. Makes sure they aren't bringing home any hitchhiking little buggies as well as knocks off any of their previous colony smell. Then I put it in a secure container to dry off. Meanwhile I clean the established rats home so it's all fresh litter, then put the new rat in there to check it out first and have a chance for a drink of water and a bite to eat. After a bit I put the other rats in and just watch that everybody is getting along okay.

You should expect to see some mounting behaviours (dominance not breeding), lots of butt sniffing and some boxing or wrestling as everybody figures out who fits where in the rat social group. Don't be surprised if the dominant male holds down any newcomers...he's just showing he's boss rat in that group. As long as nobody is biting and drawing blood, it's all good. If they get too rough I just smack my hands, rap the side of the enclosure hard and holler "BAD RATS". That reminds them I am after all...queen of the rats! LOL and they best knock off the tough stuff.

Once they are being nice again, I offer everyone a handfed treat, making sure boss rat gets his first and they generally get along fine thereafter. Males are just as affectionate if not more so with each other than the females.

P.S. Always make sure any incoming rat is absolutely healthy before putting it in with your established colony rats.