introducing 2 younger cats

Question
I have recently adopted a younger @9 month old cat from the same no kill society that I adopted my first one at 6 months ago.  They are both right around the same age, both females and both spayed and UTD on all their shots.  I am keeping the new one (Mia) in a spare bedroom with all the things she needs; ie: litterbox, food, water, toys, carrier, scratching post. My resident cat (Storm) sits by the door constantly and hisses 2-3 times a day but is otherwise very mellow.  She has growled once.  How long should I keep the new cat isolated?  She repeatedly tries to get out of the bedroom and I feel bad leaving her in there.  Both cats like I said are from a rescue and they are both familiar with being around other cats.

Answer
Pauline,

Questions regarding new cat/kitten introductions are quite common and I have answered a large number of them. I would recommend that you check out my previous answers on new cat introductions, however I am willing to offer some information on the subject. If you require more details or clarification after reading this answer and those regarding new cat introductions I would be more than happy to help you out.

So far it sounds as though your new cat introduction is going well, you've taken the proper steps to having a smooth introduction. I'm also glad to see that both kitties in question are spayed, this will help with keeping things low key. An important fact to consider when introducing new cats to one another is the fact that cats are very territorial and they tend to hate change. Storm's behavior is perfectly normal at this stage, if you allowed Mia out of her room there would likely be a serious fight that could result in injuries to each of the cats as well as any person who stepped in between them to try and stop the fight. Mia is fine in her room for now. I usually recommend that new cats coming into a household be kept isolated from resident cats for a minimum of 2-3 weeks. This isolation period serves a couple of purposes, first it allows the cats to get to know each other's scents gradually in a non-threatening environment, it allows for a quarantine period to protect resident cats from common viral and bacterial infections and it allows you to get to know your new family member and what's normal for her during this period of time. It's also a good plan to have Mia assessed by your family vet to ensure that there are no health concerns that could be passed on to Storm. Just because the cats are accustomed to being around other cats doesn't mean that they will play well together when introduced face to face too early. Due to the fact that different cats have different scents, temperaments, personalities, etc. it's best to play it safe and wait until Storm and Mia are showing signs of interest rather than defensive or offensive behavior like hissing, growling, etc. Waiting until the hostility subsides will ensure the best chance of these girls getting acquainted with each other with a minimum of fuss, there will obviously be some posturing until they figure out who sits where on the feline hierarchy in your home, however you shouldn't see outright scrappy behavior. I know that cats can be great at making their human caregivers feel guilty when they are confined, but think of it this way - if Mia was in a shelter environment she would likely end up caged at least part of the day...The current situation is temporary, she will get to roam around and explore the house soon and she has the freedom to play, eat, use the litter box, etc at the moment. In the interests of speeding up the introductions between Mia and Storm (although they're already well under way) you can feed the girls on opposite sides of a closed door which will allow them to associate each other with something positive - food. Exchanging bedding is also okay once you've ruled out feline leukemia, FIV and other contagious viral infections that can be passed between cats living in the same home.

If you think that the girls are anxious you can try a homeopathic remedy called Bach's Rescue Remedy which is a blend of flower essences designed to calm and reassure. Rescue Remedy is a very safe remedy and is used by vets and pet parents around the world to treat anxiety, trauma, fear or similar states of mind. You can usually find Rescue Remedy in health food stores, naturopathic pharmacies, some mainstream pharmacies in the nutritional supplement aisle and I believe you can also purchase this remedy online. I would recommend that you add 5-7 drops of Rescue Remedy into a clean bowl of water each morning. You'll be surprised at the difference it can make.