Before Hattie died I didn't want to get anymore just due to the stress of adding new blood. Since the circumstances have changed I am prepared. My blue laced Wyandotte decided the other day that she had a vendetta against my EE, she chased her around (they were free ranging) and even pinned her down. No blood, just being a bully. The Wyandotte is twice the size of the EE, can't run as fast so the EE was safe. They roost together and have always been ok, I think the EE was saying mean things to the Wyandotte! I had to merge three "flocks" at the beginning so I am aware of the behavior.You have a couple of things to think about with your additions. No matter how peaceful your hens seem, it is possible/likely they will test the new girls, as they establish their new pecking order. That would be one reason to think about a started pullet or even a hen, although it could still go wrong in a number of ways. I found exposure through wire eased the joining some, but even after that, when there are no barriers there are always tests to determine pecking order. It evolves, too, my red hen was Boss until my Speckled Sussex got bigger than her, then things changed again. I think she might still be top girl, but most of the SS are no longer afraid of her and won't run when she tries to make them. The Cochin, though, is able to make some of them run with the right glare and stomp, and yet she is the slowest when it comes to retrieving treats, because she's a Cochin and just isn't a darter.
Sorry, back to your flockWhatever you decide to add, you will have an easier time with integration by introducing them on opposite sides of wire, combined with free ranging them together if you are able, then when you are ready to merge them, if the new girls have not already tried to go into the main coop, put them in at night on a Friday night when you will be there when they awaken in the morning and see what happens. There will be some pecking and feather pulling, but more than a few feathers or for sure any blood drawn will tell you they aren't ready yet. Feeding treats when they are together the first few times helps, they are more focused on getting treats than on the fact there are "others" present. I found scattering scratch or tossing an apple cut into chunks made for total focus on eating - to this day!
Next month I want to start the serious search for some chicks/pullets.