Some cats never adapt to others living in the same house. Sometimes that's due to their personalities and sometimes it's because they've had bad experiences with other cats at some point in their life. I've never had a problem bringing a new cat into a house where there's already a cat, but I've read tips on doing so. The best idea seems to be to set up the new cat in a room by itself with it's own litter box and food/water dish. Don't keep it locked in there. Occasionally shut up the other cats for a short time and let the new cat have a chance to investigate it's new surroundings. It'll get used to smells of the other cats. The older cats, if let roam in the room the new cat uses, they'll get used to the new cat's smell. For short periods of time, allow the cats to interact and gradually increase the time they spend together. As I said, I've never had the problem, but many people say this works.