Nichola Farms
Chirping
Of course this may seem like an obvious reply, but in our situation we've found that the more space there is the less bullying happens. I would guess that's because there's room enough for everyone to have their own space. You said in your case space is an issue so like others have said, removing the bullies will solve the problem. Try letting them back together a week or two later and you may find that the bully is no longer the flock leader. Chickens work things out themselves on who's the boss. I've seen the biggest toughest brahma rooster cower to a loudmouth when introduced to one another. They'll usually "explain" who's the boss to each other in chicken language and after a day or two everyone's living together just fine.