When I raise roosters together, the head-rooster always develops and start crowing way sooner than the others, which remain looking like hens most of the time. I was surprised when my hens turned into roosters upon leaving the dominant rooster!
That does work, but its not that they are aggressive because of introductions, its because both think they are the lead rooster and want to stay that way.
I've heard somewhere that when a lead rooster loses a battle and gets kicked out of its place than it will become very depressed and even starve its self. I'm not sure if this is true.