Couple things:

You'd be wise to quarantine any new arrivals for a few weeks, just good bio-security.

You have to be careful putting new birds in old bird areas. There could be some fights to the death. It's best to either slowly allow them to become adjusted to each other, but not get at each other (side by side pens/coops) which I've used successfully OR

Putting the new arrivals into the coop after dark and the others have gone to roost. You have to do this without lights or rousing the old birds. Many say this method works, but I've never tried it.