Hi, I was in a very similar situation a few weeks ago. I had bought four chicks, and then it turned out that one was a roo, around age ten weeks. The place I bought them from agreed to give me a new hen to make up for the roo. They let me keep the roo, since I had found someone who wanted him (and not to eat). I got a new hen by herself, who was a week or two younger than the four I had. First I had her live alongside the others, in a separate run, for about ten days. Then I let them all in the same big space one day, but let them free range and gave them exciting treats to focus on. They were so busy with all that, that they didn't bother to chase the new hen too much. I had her sleep in her separate little coop a few more nights, and repeated the free range with treats all together a few more days. She got chased away some, but it wasn't too bad. Then one evening when they were all settled on their roost in the coop for the night, I put her on the roost with the others, and it worked. I made sure to open the coop right away at dawn, so that she would not get pecked on in the small space of the coop. She is stil at the bottom rank, but not getting pecked. The more space, the less fighting! Good luck!