As promised, some thoughts on various approaches to introducing a new cat or kitten to an established cat-bearing household.
Frequently the preference of men and boys not entirely fond of cats but appreciative of a good scuffle, this method involves unceremoniously dropping the new comer into the midst of the established feline family. Trouble almost always ensues and fur often flies. All manner of bad manners are possible, including inappropriate urination, which I broadly define as anything outside the litter pan or designated equivalent. Additionally, the spread of parasites, like fleas and intestinal worms or communicable diseases ranging from upper respiratory infections to fatal leukemia or immunodeficiency viruses is possible without proper quarantine and testing. And, it's just not a kind thing to do to a cat or kitten already stressed by a new environment, new food, new smells, new people....or to the established residents who thought they had it all figured out. Don't gamble this way.
Never the Twain Shall Meet
At the other extreme is complete isolation of the new cat from everyone else. Sometimes this is necessary for reasons of temperament or viral status but otherwise not required.
A Happy Medium
Aren't we all looking for the "just right" Goldilocks solution?
What I usually recommend to clients and actually tried myself with the recent addition of the best little grey tiger kitten ever, aka Scout, (her story's here), is the following:
1. Test for Feline Leukemia and Feline Immunodeficiency Viruses. A negative result is great but bear in mind that recently exposed cats and kittens may not actually test positive for up to 12 weeks. There are two good options. The first is to test and re-test in 12 weeks. The second is to confine and quarantine the new addition for 12 weeks and then test once. This offers some assurance that the resident cats aren't exposed to a potentially deadly disease and you don't get your hopes up for a decades long relationship. (The management of a kitten or cat with a positive test for either virus can be controversial. Suffice to say for now that a single positive test in an otherwise healthy feline is not an immediate death sentence.)
2. Treat for parasites. Like puppies, kittens can acquire intestinal worms even before they're weaned. In the case of an adult cat with an unknown history, deworming and a stool check for intestinal worms is good sense. Any flea & tick infestations should be addressed so your new pet doesn't bring along pets of her own.
3. Update vaccines. Depending on the age and known history of the new feline, do the best you can to ensure good health and immunity. Consider boostering the upper respiratory complex and feline leukemia vaccines for any current residents, just in case.
4. Create a safe zone. This was key in making Scout's transition smooth. It was also facilitated by an "extra" bedroom thanks to a non-resident college sophomore. Scout moved into her own space, complete with food & water bowls, an exclusive litter pan, a pillow & comforter-laden bed, and two big windows featuring birdsntreesnsquirrels oh my! That first night home she circled the room twice, (counter-clockwise we noted for whatever reason but suspect may have something to do with our cats having a tendency to be left-pawed), then settled in for dinner and a nap. Three months later, Scout still naps on that bed after her mad morning romp. (The college sophomore is resigned to sharing her room. I console her by noting it's better than a dormitory.)
5. Gradual introductions. The sounds emanating from the bedroom , (primarily the crunching of kitten kibble and the occasional crash of a gravity check), not to mention the loathed existence of a closed door, riveted the attention of Minerva Jayne who appointed herself hallway sentinel extraordinaire. Similarly, they roused the suspicions of Holly who sulked behind a poofy tail which I attribute to her remembering the assault upon her sensibilities when Minerva arrived a few years ago. After a few days of auditory acclimation and the occasional glimpse, paws began reaching under the door from both sides.
6. Cat nip, treats and toys. Many kittens aren't fazed by cat nip but both of my older girls find it irresistible. The initial frenzy of rolling is soon followed by a mellow-cat kinda vibe, the perfect state of mind for an introduction. Pairing the new comer with some tuna water or special treats can create an aura of good feeling, or at least distract and lead to an intense grooming session. (I did refrain from dousing Scout in tuna but the thought did occur to me...) New toys also attract some attention and enrich the environment.
7. Sufficient resources. Separate food and water dishes are a good idea. Everyone should be able to eat in peace, cats included. Enough litter pans, (i.e. the number of cats plus one is ideal), in enough places that there's always a "vacancy" if needed. Laps for napping and windows with sunlight are important too.
8. Accept the inevitable. It's not going to be entirely conflict-free. Someone will get too close and invoke a hiss. Someone else will say something worse, leading to some paw swipes and potential injury. If hostilities escalate or simply don't resolve with time, consider backing up to step 4 or consulting your veterinarian for some additional stress-reducing ideas. I'm a fan of Feliway, especially if there are any issues with proper litter pan use.
And that's the recipe that worked for me. Good luck on your own new additions, especially if there's something soft and meowy under your Christmas tree...
Speaking of which, I'm sure the resources here will happily find a kitten or cat to own you.