Okay, buckle up buttercup, I'm going to dump a bunch of info on you, lol. I have volunteered as an adoption counselor for the past 18 or so years at a cat shelter, so I've got some opinions and suggestions for you.
First, are you adopting for yourself? Always look for the cat that best serves your needs; you will never be guaranteed that the cat you choose will be welcomed by your existing cat.
Is he a lap cat? If he is, I would steer clear of the cat that must be sitting in someone's lap to be happy.
I would suggest looking for the cat that interacts with you and the other cats in a positive manner- in other words, don't choose the timid/frightened cat, they are often targets for a bolder cat. Also, don't choose the cat that is a brat or aggressive toward the other cats; they often will do the same in a home and you don't want your existing cat to become a punching bag for the newbie. Pay attention to what meds the cat is on. At our shelter, we have several cats on Prozac or similar behavioral drugs because they are aggressive toward other cats without meds. This is not the type of cat you are looking to adopt. **Please note our shelter is 80% cage free-cats are isolated for a 2 week quarantine, for medical issues or may be placed in suites with limited other cat contact because of medical or behavioral issues.
Talk to the volunteers. They will have an idea about who would work well with an existing cat. I would tend to steer you toward younger adult males; imo they work best as new kitties where there is already an adult male. Females tend to be territorial, even if they are not the first into a home.
Unless your boy is very, very active I would suggest an adult or at minimum a teenager (9-18 months old). If you are dying to adopt a kitten, adopt two so that your boy isn't pestered to death by the new kid.
Once you have settled on the new cat, create a "safe room." This is a room with doors you can close that is not your current cat's favorite place (if he sleeps with you, your bedroom isn't a good safe room, if he helps out in the office, then that isn't the right room, etc). The safe room is set up with everything the new cat needs- food and water in one area and litter box far away but in the same room. The new cat will be taken straight into the safe room (no face to face greeting, even through the carrier) upon arrival, you walk in, close the door, open the carrier and leave the cat alone. After 30 minutes or so, go quietly back into the room. Hopefully, the newbie will come out and greet you. The newbie stays in the safe room for several days- you want to make certain he is eating and using the litter box, but also that he hasn't brought anything contagious into your house. Your existing cat will know there is someone new in the house and this is key-the best things in the world occur outside the safe room- if your cat loves treats, they magically appear; if he is a fan of the laser light, playtime takes place outside the room; if food is his thing, a bowl of kibble shows up outside the door. In the meantime, you are visiting with the newbie multiple times a day, you are sitting and reading or watching tv, checking the DIS forums, whatever, with the new cat.
Once you know the new cat is healthy, that he's using the litter box and eating and your existing cat isn't hissing or growling at the door, you can open the door to the safe room and allow the newbie to explore your house. Keep the safe room door open so he can run back if he gets scared. You must be home for this part of the introduction. If you leave the house during the next few days, you have to put the newbie back in his safe room; you do not want a cat fight to start while you are out. After a few days, if the guys seem to be getting along you'll be okay to leave them or short periods without supervision. If there is growling or any aggression from either cat, step back to the safe room steps and start over. After a week or so of the new kitty being strictly in the safe room you can try to reintroduce them. Do not rush this process; if at anytime there is tension, step in and separate the cats, placing the newbie back into the safe room.
After every thing is great, when the new cat is free roaming in your house, create two feeding areas and two litter box areas so that one cat doesn't guard the resources. I've seen so many situations where adopters didn't add bowls or litter boxes to their house and things went sideways after a few months.
Feel free to message me if you have any questions!
I think, like people, some animals end up with crap DNA through no fault of their own...or anyone's really.
