You can't add roosters that haven't been raised together without fighting. A lot of times they'll eventually go at even if they grew up in the same cage. In a large flight pen or a cage that has a lot of square footage multiple roos are ok, but in most small cages they can't have their own territory so they'll never stop fighting over what territory there is. Try to have 6-7 hens per rooster as well, so they have plenty of girls to distract them. Honestly in the size pens most people are keeping them multiple roosters isn't very feasible. When you have them in a couple hundred square feet those problems become much easier to deal with and much less frequent.
