a. you have too many roosters.I have 6 roosters. 1 got injured somehow before bed one night. Really bad limp, but no visible issues. Decided to separate him for the night. The next morning I put him inside the coop in a corner. Another rooster seen that and went right in to show his dominance. I kept him isolated a few days until he was a little better. I have heard the time to re-introduce chickens is at night. I tried that, and didn't seem to have as many issues as far as fighting.
b. it's virtually impossible to introduce, or reintroduce mature roosters together.