Do they do this in the run or are they picking on the others when they are in the coop too?
When I merged my 5 girls (3 EE & 2 Light Brahma's) with my original 10 (they are only 2 & 4 weeks apart in age), I had to fence off an area in the run, just for the 5 little ones, because the older 10 were so mean to them. I used a roll of hardware cloth and blocked off an area, so they could see each other, but where the 5 were still safe. 3 weeks later, I opened one side of the fenced off area, so that the 5 could go out with the others, but could go back to the safety of their fenced area. The older ones never intruded into the fenced off area.
At night, the 5 younger were still being housed in a large rubbermaid container, and I did this for 2 weeks, then took it out to see what would happen. The younger 5 found the roost & stayed up there, the older 10 were still puppy piling in the corner of the coop. 2 weeks went by before the older 10 joined the 5 on the roost. But the younger 5 would huddle on a corner, because they would still get pecked from time to time. Never had blood or feather loss, but a few squawks & screeches from time to time.
Even now (they were merged 4th of July weekend), the 10 stay separate from the 5 younger. The 5 younger go to bed first, and are the last to leave the coop in the morning.
How long have they been together?