Despite common misconceptions, you don’t just “throw cats together” and let them “work it out on their own.” Doing this can create DISASTEROUS results. Cats are territorial creatures and you should tread carefully when bringing a new kitty home.
SLOW INTRODUCTION is the key.
If you know you are going to get a new kitty, one helpful tip is to prepare the existing pets in the home for the new arrival. Existing cats and dogs may feel threatened once the new comer enters their sacred domain. Calming collars, calming sprays and plug-ins (such as Sentry Calming Collars, Calm Keen or Feliway plug-ins) can help create a soothing environment prior to new kitty’s arrival and reduce stress. A local vet recommends doing this about a month prior to bringing home the new cat, if possible. Your vet may also be able to offer you other calming products they can get for you right there or you can go to Amazon.com with the vet’s suggestions. Our fosters have had great experiences with Calm Keen but be sure to consult with your vet before you use any of these aids to make sure it's appropriate and won't conflict with any current medications or special diets your pet may be on.
Pick a separate room you can safely close the door and keep the new cat in during the introductory process. Keep things the new cat can “own” in there so they will feel secure. Things like the litter box, water bowl and food dish are standard but you should also include some scratchers, toys and a blanket or towel for them to snuggle up with. (You also want the new cat to scent these items up to use later for scent swapping.) Once you have base camp for the new cat set up, you are ready to look for your new family member. Now it’s up to you whether you think the new kitty should be girl or boy, long haired or short, petite or large. The one thing you want to try to do is match the personalities in your home. If you have a relaxed, chill senior cat, a hyper active new kitten might drive them crazy; however, you might know your cat and know that the calm kitty would have the opposite affect and would help the kitten chill out. Use your best judgement and try to think what might be best for your existing pet(s).
Once you get the new cat home, take them to their new room and spend some time with them. Make sure they feel calm and safe and know this area is theirs. Then go to your existing pets and love on them. They need to know they are still the KINGS and QUEENS of the home throughout the process. Remind them they are NOT being replaced by the cute newbie in the other room. You will want to start scent swapping. Take things that have been scented up by each cat and bring it to the other cat. You can give them treats or food so they associate good things with this “weird new scent.” You will also want to start feeding them near each other, on opposite sides of the closed door. (You want to start with it closed so they can’t see or get to each other.) As they start getting used to each other, start pushing the challenge line, a little bit. You don’t want to do too much at once. Open the door a crack and see how they do. If the cats keep eating and don’t care, well you’ve done well. Push a little bit more each day and see how they do. Eventually, the door will be all the way open and the cats will be fine. If you run into any hissing, peeing in the wrong place or other negative behaviors, you know you’ve pushed too far and need to set the bar just a little further back and start again. Every cat will be different, so there is no set time frame as to what works. Personally, a volunteer had a senior cat and a young adult. They then adopted a 6-month-old kitten. The senior cat was just fine with the kitten within a week. The young adult needed about a month and a half before she was okay with the new kitten. We know of others who’ve said it took 3 months for their cats to be okay around each other. You will just have to watch for the signs and adjust as necessary.
There are many behaviorists and websites with great info on this subject. Here are some links to help you:
www.JacksonGalaxy.Com Look for Cat Daddy Tips and Behavior Tips
Introducing Cats to Other Pets:
Introducing Cats to a New Baby: