I have.... 9 male rats, 7 are breeders, they all rotate in and out of the breeding racks and have NEVER fought for blood.
They all get along very well. All rats have to reestablish the pecking order when reintroduced, but I have never seen them actually fight. They squabble, they might be loud, but they have never fought for real.
I find the females are actually a lot more bossy than males and will tussle more often.
My males are all peachy!![]()