I got 15 baby chicks at Christmas time. I had issues with one chick being a bully. I found that she needed more space than the others. If I isolated her from the others she would complain loudly and non-stop. If I gave her one or two friends, she was fine and no bulling occured.
The one chick NEEDED more space than the others.
When the chicks were about 8 weeks old, I tried to put them with my two 13 week old pullets. THAT DID NOT GO WELL. The two older pullets picked on ALL my baby chicks. Pulled tail feathers, chased them, and in general terriozed the younger pullets.
I left the younger pullets in the big run / coop and took both older pullets and put them in a time out together. Almost immediately, the younger ones were happy and active. The older ones seemed to be fine TOGETHER in their timeout.
They have been in timeout for almost two weeks now. I haven't tried to put them back together yet because the younger ones are still smaller. Also, everybody seems happy like they are !
In both cases of time out the chicken did much better with one other bird than with no company.