I have five roos, which were all raised together from babies. They get along reasonably well, although as they have grown up their relative rank in the flock has changed. The Buttercups, who got along great with each other, were the first to develop roostery characteristics, so they were dominant for a while. Now that the late-bloomer Cochins have grown up a bit, they rule the roost due to their huge size. They do challenge each other, but I've never seen them do more than fluff out their feathers and run at each other--then one of them backs down quickly, it's not like they really fight. Then the loser gets up on the highest perch and crows to show that he is still a tough guy, or sometimes one of the girls intervenes. They are fairly cooperative when it comes to chasing away predators, herding the girls back indoors in bad weather, and clucking warnings when they hear a hawk.
Don't know how they would react to a new adult roo, though. Probably they would not be happy, especially the Cochins who are HUGE and very protective of the ladies.