The older chickens we acquired have pre-developed habits that are bad. In order to protect our little ones we keep them separate. I tried to introduce them and it didn't go well.
We are in the process of creating a permanent home for the big chickens and the little ones will move into the chicken tractor. Once the little ones have grown (11 ladies and 1 cockerel) and are laying then we will bid farewell to the older chickens.
I had nightmares that night. I thought for sure I would go out and find her dead in the morning. I felt so guilty and was ready to throw in the towel with the animals because I really felt it was my fault for forcing the chickens together and expecting harmony. Next morning I went out and she was doing much better. And now, a week later, she is totally back to normal except for her bald head. Poor thing.
Taylor enjoys the chicks while the big chickens look on from the locked chicken tractor. We just haul out the chicks in a cardboard box and corral them up in the evening and take them back to the brooder.