Well, you can try it and let the chicks have places they can go the older ones can not. Usually it's best to add birds when they have grown to about the same size. I'm dealing with pullets who are 13 weeks old and full size hens and chicks with a mama, all is going well but they can not stay in the hen house together, we have to separate the pullets and others with a dog crate at night. They free range during the day with only minor scraps, but I would never chance it in such a small space. You may find dead chicks in the morning. When adding any new chicken it's going to be a gamble, I knew this when I got my original 5 and thought I would never add more, lol silly me. I gave 2 to my mom and wanted at least nine to go in my enlarged coop, it's been a pain in the arse, but it's coming along. We just have to put pullets away at night in a dog crate ( p4 of them) chicks go in the coop with mama, and all is well. We allow free ranging from twelve noon to around 6:00 in the evenings to wander around, eat grass and do what chickens do. We have a large pen, we bought a large round dog pen/ kennel that fits in the large pen and they go there to roam around in and at bed time we put them in the huge crate. Chicks and chickens go into the coop on there own and it's good night.