Okay…so I just put in a call to a friend of mine who is an animal expert. She has fostered over 200 cats in a 10 year period (and no, not in a “crazy cat-lady” way), she has several herself and also works at an animal clinic. Here’s the scoop:
Do not, in any form or fashion, disrupt your resident cat. It has already been established at “ruler of the domain” and will basically be in control of this introductory phase you are undertaking. The new cat has already been uprooted and will be somewhat/very stressed (depending on the cat). Pick a room of your house that is litterbox-appropriate and put the new cat in there for a couple days. Not too long, but long enough for the resident cat to smell the new cat’s scent and get used to the new presence. After a couple days, put the new cat in a pet carrier and bring it into whatever room will allow your resident cat to sniff, hiss, stare…or whatever the resident decides to do. Don’t “schoo” the resident, basically just ignore any aggressive reactions and just allow the process to take place. You’ll want the leave the new cat in the carrier and allow it to sit in the open for a few hours each day, probably for at least a few days. Bring it out once, twice, whatever you interpret as appropriate according to the behavior of your resident cat. If there doesn’t seem to be much conflict, you can even speed up the process a bit. She says she usually does this over a period of about 10 days or so, but that it’s well worth the time you put into it. Apparently, she’s never had a fight or any random peeing or other aggressive behavior. Oh, and she says as long as they’re spayed/neutered…gender doesn’t matter one little bit. She’s never had an issue with mixing, matching, etc.
Let me know how it goes!