I'm in the process of the exact same thing. I had 3, 4 year old hens (2 polish and a laced wynadotte bantam) and then got 4 more chicks (welsummer, 2 blue andalusions and a speckled sussex) this year, my husband got a turkey so now we have her/him too with the chicks and they are all doing fine and are about 8 weeks old now. Then my neighbor, who no longer have chickens and are retired snowbirds found a chicken in their yard that none of the other neighbors say is theirs. Long story short we have her now too. We think she is a red sex link thats probably about a year old. We've been integrating her with are older girls and so far so good. They are wearily excepting her and it's actually making our little bantam move up to being able to sleep on the same level as the polish who prior to this would make her sleep lower. Oddly no one uses the top perch-honestly I don't know if the polish even know it's there, they are derpy birds. So our new girl, Missy has been sleeping up there and it's like she doesn't exist to the other 3 if she stays up there.
We have an old coop and my husband just built a new one. The main coop is 4 x 8 like yours, though the entire building is 8 x 8. The other half has a brooder, storage shelves up high and in case we need to separate the turkeys sleeping quarters ever. The inside is 8 feet tall, so we have lots of height. The older girls have not been put in there yet. The chicks are all in the brooder side, though since they are getting big I opened the brooder so they have the whole 4X8 space. The two spaces are separated by hardware wire. We will probably add a run sometime this summer in case we ever decide to go on a v acation (thats a big fat maybe, we really don't go anywhere). Otherwise eveyrone just free ranges the fenced backyard all day. The new coop was built where our old veggie garden was so it does have a little picket fence around it that are older girls don't jump..cause they are lazy mostly I think. That so far is working out since I can let the babies out with human supervision in the smaller garden area and the older girls can see them. The plan is to add the older girls to the coop and leave the new ones on the brooder/supply side till we are sure everyone gets along. I figure they will be sleeping in the same place..just separated by wire in the meantime.
I'm not sure if I'm over thinking or under thinking it all. But when building this I did not plan on the extra one year old hen or the turkey, so hopefully all goes well. I also don't have a covered run right now, though if it rains they usually don't care, or if they do they go through the dog door and come in the house (though all they make it to is the mud room since I keep the door shut in there so I don't have dirty dogs running through the house) There is the border collies dog kennel in there and the two polish like to sleep with him sometimes when the weather gets cold.