Keep in mind cats are territorial creatures cats believe a certain part of the house, the yard or even the neighborhood is theirs.
If another cat enters their space, fur will fly. Most aggression between cats is between non-neutered males the answer is early neutering to nip this problem.
Ever wonder why other cats in the house are aggressive to the cat that just came from the vet its the odd odors on the cat. Rub diluted tuna water down the backs of all the cats they will be too busy licking and grooming each other to fight.
How to pick a new cat that will blend easily with the other house cat first, pick a younger cat, 4 to 7 months old, of the opposite sex. Next, separate the cats completely for two weeks before any mixing.