Introducing a new cat
One of the biggest differences between dogs and cats is their ability to get along with their own kind. Dogs can figure out who they're going to get along with almost immediately. The pecking order is set quickly and the submissive animals happily concede to the alpha with wagging tails and big grins. Cats on the other hand, have to be slowly introduced to a new cat, otherwise hair will fly.
The first rule of thumb when introducing a new cat in the home is to make sure it has a clean bill of health. Assume there will be physical confrontations. Even cats who are buddies fight, so make sure the new cat is disease free.
Segregate; use separate rooms and amenities, then slowly introduce at meals for a few weeks. This is a time consuming task, but if you've ever lived with two cats who wanted to kill each other, you know, this is time well spent.
Cats do well alone, but siblings and mother/kitten combinations are known for their camaraderie. A solitary cat can become used to another cat, even if they're spoiled rotten. The key is to work slowly and be patient. Confine the new cat to a room. For the first few days and allow them to become familiar under the door. Swap the rooms a few times for a few hours so the new kitty can explore the house and smell the established cat. Not too long though, you don't want your existing cat to feel like it is being punished.
After a few days of segregation, feed the cats together (perhaps with something yummy, like canned food). Make sure the cats can see each other, but have some distance from each other. Once the new cat is done eating, return it to its room. Every few days, move the dishes closer. Never leave them unattended at first.
Once feeding time seems to be calm, you can add brushing to the regime. Make sure you position the cats so that you can remove one of them quickly, ideally on either side of you. They will begin to associate being together with good things. All their little qualms go away when a brush is around.
Final recommendation is one that should be done from the beginning, add two more litter boxes. Always have one litter box per cat. The extra one is for territorial disputes. Be careful where you place the litter boxes; the potty can be an ideal place for ambushes.
Alleviate any growling or hissing by moving or segregating the cats. This process can take a month or more depending on the temperaments involved. After they become accustomed to each other, watch out for disputes. If one is smaller or older than the other you may need to establish expectations. Cats are smart. If you always let your 14 year old have the window, eventually the younger one will give up the seat when it sees the senior headed its way.
Be patient and when in doubt, don't put the animals in a situation where they will be stressed. Cats can even become friends if introduced with a minimum amount of anxiety.
