It really depends on the roosters. We seem to have a lot of trouble with the Marans... hmmm... yet another reason to totally get rid of the Marans...
We have a few bachelor pens. We don't like to keep them by themselves unless we have to, we'd love to keep them all together but again with the fighting. Some tips:
Don't add just one rooster at a time to a bachelor pen. He will get picked on relentlessly - if you add two at once they'll get half the bullying each, much better than 100%!
Try to not have them in eyesight of hens - and DONT house them right next to hens or where hens free range.
If you're going to range them, don't let them range near hens. This will trigger fighting for sure.
Pull a bully out. Sometimes interrupting the fighting and grabbing him up will discourage him, you can try, but sometimes they're just bullies. Try to find a companion he gets along with and house them in a separate pen.
Sometimes it's best to introduce everyone either at night or while on free-range, as long as there are no hens around. That way they have enough room to move about.