I have 13 easter eggers from 8-10 weeks old that have been raised together, and 5 rhode island reds that are 6 weeks old - these were raised in the same room with the other chickens but in a separate brooder. The easter eggers have been in the coop for a couple of weeks now, and I am trying to introduce the rhode island reds.. it isn't going so well. First, I put my chicken tractor next to the run for a couple days so the ee's could see the RR's but not get to them. Then I moved the RR's into the coop with the ee's yesterday. I expected some pecking but the EE hen who I believe is in charge is relentless. The other roosters/hens have done some pecking and chasing, but nothing excessive. This hen will not let the RRs outside of the coop, and she sometimes goes into the coop to peck them anyway. When the chickens go to roost at night in the coop, she kicks the RR's outside and won't let them back in! I put the RRs into the coop last night because they were too afraid to go in, and they were fine all night because the other chickens were asleep at that point.... but as soon as morning came they were getting harrassed again, and now they haven't left the coop all day while the other chickens are outside. Most of the EE's just want nothing to do with them, but that one EE hen goes out of her way to be nasty. Since this is my first time raising chickens I don't know if I should be worried about this behavior or let them work it out. I am afraid she will draw blood. I am giving it time for now and watching as closely as I can - I don't know how long it should take before they are accepted into the flock.
Separate the aggressive one for a few days or a week. This will allow the others to set up a pecking order, and she may not be top girl any more when you reintroduce.
When you're ready to let her play with the rest of the group, you might try putting your 2 or 3 most dominant EE in with her. Hopefully, they'll turn the tables on her, and she'll bond with them before rejoining the rest of the group. IMO, EE are a very docile/submissive breed, and RIR are much more aggressive/dominant in general. It is normal to have one hen in a group of all hens that will rise to the top. My Alpha RIR is sure to let the others know that she is the boss. She won't let the EE eat until she decides that they can, but will share food and space with her BFF. There will always be an Alpha, and there will always be an underdog who is like the little kid sitting alone on the playground with no one to play with.