I am most concerned about my older cat. Has anyone ever introduced a kitten to an older cat, especially a pedigreed that is this old and has never been around any other animals? My mom works from home and has customers, so every once in a while someone may bring their big dog in our house for a bit and my cat seems completely unaffected. However, one time, maybe about 7 years ago, a friend brought her new kitten over. As soon as my cat saw my mom hold that kitten (as he is most attached to my mom), he hissed at both my mom and the kitten.
The others on the forum have covered enough about the kitten for me to not need to reiterate what's already been said, but I also want to bring up the possibility that now, when your older cat is clearly on his last legs, might not be the best time to bring in a new family member, especially not one with the activity level of a kitten, paralyzed or not, and especially not one that requires a special amount of attention when the cat has proven to have plenty of issues with jealousy. I am assuming that with paralyzed hindquarters, this cat is going to need help eliminating, eating, etc. and I don't know if it's entirely fair to add that stress to a 16-year-old cat that has a limited time left.
That said, if you
do decide to bring home the kitten, introductions need to be done slowly. For this next week, either spray your house with Feliway or get several Feliway plug-ins. Another option is to additionally bring a piece of cloth or blanket from the kittens' kennel home, and to bring something from home to the kitten, so that they have a chance to smell one another without their actual presence. However, I have never utilized this step and introductions have always, for me, gone fine. I do like to use the Feliway, though, and continue to keep Feliway on hand to keep the cats away from scratching certain pieces of furniture etc. It's definitely a good spray to have on hand.
When kitten actually comes home, give the kitten her own small space. Since the Siamese is possessive, I would say give the kitten a space like the laundry room or the bathroom, not the bedroom or anywhere the Siamese might have claimed as being for him or for his people. If the Siamese does not approach the door, then leave it alone, but if he becomes interested as to who's on the other side, that's when I would start associating the smell with good things. Does he have a special food? Or a special treat? Feed this to him but only when he's by the door. Let the door become less about the cat on the other side and more about the fact that that cat has brought good things with them! Do the same with the kitten. Kittens are a lot more "ooh, hi, how are you?", but it's still good to reward the kitten, too, for accepting the strange cats' scent.
After I do this a few times, I normally open the door slightly, but so that neither can get in or out, and allow them to see/sniff each other that way. I believe Jackson Galaxy at one point used a baby gate for this, but baby gates have never stopped my particular kitties, so they might not be fool-proof.
But if the cat responds positively, then I'll start feeding them where they can smell
and see each other, and a day or two after that, I'll usually let the little ones out, feed the cat a big heaping pile of his favourite food, give the kittens their own bits of food a few feet away, and then watch to let them work it out. If I see signs of tension, that's when I bring in toys and engage everyone in a game. This will be a longer or slower process, depending on the cat. My sister's cat accepted my two kittens within a few days but he's always been really good with kittens. When we fostered, it took my older cat about two weeks before he could finally be in the same room with the kittens.
This is, however, assuming that after all things considered, you think it's still a good idea to bring kitten home. Given your situation, it might be better for everyone to euthanize the kitten and allow your Siamese to live out his remaining time in peace as the only cat. I think it's important to assess the fairness of bringing home a new kitten to a cat who has been diagnosed with kidney failure and who is deteriorating and who has always been an only pet. Something to think about. Good luck with whatever you decide, though.
