Are you the Alfa in the flock?
I am and my two roosters and all the hens know it! In a flock it is the job of the Alfa to maintain order... When I am not there.. My #1 roo does the job for me or some of my older "crabby" dominate hens pitch in to keep order.
I have had great sucsess intergtrating flocks, and new birds... Matter of fact we just finished introducing a young Silkie into a flock of 25 Marans, Ameraucana and EE! The little silkie is one of the gang now for the most part..
OK:::: How we do it:::
What works for us is to spend time with the flock, especially at feeding time. When you see another bird be aggressive, pick it up hold it firmly on its side tight to the ground right under the chicken it was picking on. The chicken it was picking on may peck the aggressive bird once or twice maybe not... Be fast with grabbing the "naughty" chicken, continue to hold it on its side on the ground until it relaxes and until the picked on bird walks away. Repeat, repeat and repeat.
Also give the picked on birds treats and do not let the other chickens in to share, shoo them away. Remember you are the Alfa of the flock, reestablish the pecking order.
As far as roosting that is far more difficult to police, you can pull the aggressive bird off the roost and toss it on the floor, while placing the picked on bird on the roost... But ultimately they have to want to roost together... I would make a bigger roost, and concentrate on the daytime.
Another idea is to isolate the most aggressive bird for a few days.
Good luck
ON