First of all, do not get a kitten. Kittens are too rambuncous for a mature cat like yours. When I bring foster kittens into my house, Tyson, my 10 year old, hisses at them to keep them away, Mandu, my Himilayan, totally ignores them, and Orange Julius bops them on the head. But they're fosters and they're not staying, so it doesn't matter. When I had my son's cats while he was moving to Florida, I kept them in the bathroom for about two weeks, then let them wander. When Mandu felt Bailey was infringing on her territory, she would would chase Bailey into the bathroom, then sit outside the door to make sure she stayed there!
Get an adult cat. Two to three years old would be sufficient, perhaps older if you find one you like. The older a cat is, the better the chances that you're saving it's life. Then they have to be introduced properly. You can't just throw them in together. Put your new cat in a separate room, the bathroom works well or a spare bedroom, with food, water, and a littler box. Let them meet under the door. Your old cat will sniff under the door and so will the new cat. They will most likely put their paws under the door too. Your old cat will also smell the new cat on you from when you go into the room to play with it. Give them about two weeks to adjust, then open the door and let the new cat introduce him/herself to the rest of the house and your old cat. Best of luck to you.