Success stories? Intro older "single" to resident Duo?

My husband's father can no longer care for his 13 y/o cat. The cat is a sweetheart with people but has been an "only" his whole life. My husband wants to bring him to live with us. We have 2 kitties who are good buddies.

Anyone have this dynamic and it worked? I just don't see how we could take the "only" and not have utter chaos.

Any help/successes appreciated!

I have a senior, she has been with us almost a week. She is isolated upstairs for the time being. Because of her special medical problems she will more than likely only be an upstairs cat, but you can successfully merge the cats together with little problem if you do it gradually-

Introducing cats