I only have one main hen house and this is an ongoing problem. Mixing new birds with the regular gang can be difficult. Basically all you can do is put them together, make sure there is plenty of roost space for everyone and cross your fingers. If roost space is short, you make a problem worse.
The 'new' birds always seem to roost in the nest boxes and fill them with poo, they are pecked and chased off the roosts. I sometimes cover the nest boxes with cardboard so they have no choice but to get on the roost and after a while the pecking and shoving dies down.
One other thing, I make sure there is NO LIGHT at night when I mix old and new. I find the more light the more active and the more aggressive the hens are. For the safety of the new guys, no light makes the job of integrating a little eaiser. In my opinion.
I also never introduce birds that a very young or small, they just get relentlessly picked on. I wait until young birds are fully feathered and almost their adult size. It's still tough and there are still fist fights, but it can be done. Good luck!