As to your whole set up, do be sure you measure your whole set up, because what can seem like a lot of space when the birds are smaller can rapidly become too small of place when your birds grow up. This can cause a whole lot of ugly behavior. Your birds have a lot of growing to do.
I would be quite wary of that rooster. He is rapidly getting to the age, where he can be a real and possibly violent problem. Roosters in my experience, take a lot of chicken experience. They can ruin the whole chicken experience for a lot of people. He is your oldest bird, and roosters mature much earlier than the pullets. He is going to be bigger and wanting sexual favors long before the pullets are ready. Due to his size, he will bully them and attack them. He may become such a bully, he may try and attack you or your family. You have years to do this hobby, think on adding a rooster next year. If you can rehome him or cull him, that would give you a bit more room to add 2 more pullets.
As to your original questions, I would not add a single bird. EVERY ONE in the flock will attack her, no matter how nice they seem now. It is hard. When you add a couple of birds, well all that chasing and pecking is tiring, and it spreads it out over two or more birds. The older hens will give up sooner, and the new bird has a partner. Adding more birds is much easier.
Mrs K
