Here is a article on introducing one set of cats to the other.
http://www.piftails.com/main/article...ingnewcat.html

Re. Keeping one set of cats inside and letting the others out, all depends on your house layout.

Bear in mind that coming from England all ours are inside/outside but when we introduce a new cat to our home (after they have been isolated etc). We are in the same boat as you.

ie. All the original cats want freedom to go in and out but the new cat must stay in for some weeks to bond with us.

There are several ways you can do this but the easiest is to install a cat flap and leave it locked in the 'in only' position and let his cats out when they ask. In this way his cats always can come inside when they want or need too but yours will stay inside.

Caution. Two out of our seven cats have learnt how to open the catflap inward using their claws in the seal and flipping the door inwards and other their heads. But those two Milo & Treacle are particular bright cats, most cats won't figure it out.

Good Luck