advice on bringing new cat into household

I have 2 cats Stretch and Baby. Stretch is a male who will be 18 years old in February and Baby who is female turned 17 this August. I would post pics, but I’m not sure how.
Stretchs kidneys are beginning to fail and I have him on a low protein diet. Other than some loss of muscle mass/ weight loss, you’d never know anything was wrong with him. He has a great appetite & energy and my vet thnks he acts much younger than his age. He is given a medical checkup every 6 months.
Baby has high liver enzymes, but medication (Denamarin) is keeping that in check. Basically she’s in great shape for her age and gets regular checkups.
An aquaintance of mine has a male cat Bootsie, approximately 6-7 years old. This cat has never been properly taken care of IMHO. I’ve had to beg him to let me take Bootsie to the vet to get his nails trimmed because his nails are so long the cat can hardly walk.
Anyway, it’s come to a point where this aquaintance cannot personally and financially take care of Bootsie anymore. Everyone involved has only one solution when it comes to Bootsie; Take him to the pound. I doubt this cat will ever be adopted since he’s older and the pound is overflowing with other cats. Bootsie needs to be placed by the end of the month.
Me and my BF volunteered to take Bootsie, but I’m concerned. I don’t know how to introduce him to the household. I don’t want to stress out my other cats, especially Stretch. If bringing Bootsie into the house will affect Stretchs health, then I don’t think I should do it. He is my number one priority. Baby might be upset at first, but I think she would have an easier time adjusting to a new cat.
Does anyone have any advice on my situation? What is the best way to introduce a new cat into the house? Should I not risk doing it all all since my other cats are old and have health issues?
Also, Bootsie has never been neutered and if I keep him I would want to get that done. Do cats have a harder time adjusting to being fixed the older they get?
Any and all advice is welcomed.

keep cats in separate rooms with shut doors. take objects that have cats scent (rub towels on them) between new and old cat locations. let them get used to other cat odors. later let them have visual and smell contact through a barrier (metal mesh).

O Feliway you are great!

You can find it in any pet store. There are sprays and plug ins and it’s like magic. The cats smell it and they get a “hey, there’s a groovy cat who lives here” vibe and everyone calms down.

Good luck!

This page isn’t a bad start, and much more than I care to type out. The only things I disagree with are the recommendation to bathe the cats in the same shampoo, that’s just silly, and the other thing is the length of time, I’d give it longer. The recommendation I usually hear is 2 weeks before any face-to-face introductions. Overall, that page should be very helpful, though.

I have four cats, and I used the 2-week initial “quarantine”, then similar techniques to the referenced page for each of the three additions to the first and oldest cat. I can’t say any of mine are best friends, but they tolerate each other enough for it to be a mostly peaceful household.

Also consider a wire cage (kennel) that you can use to put the new guy in where everyone can see and smell each other and yet, no one can touch! I’d give them a few days behind closed doors, then start the crate for a few minutes at a time, longer and longer, over the next week or so. Expect lots of growling and gnashing of teeth, most likely with the younger cats. My 17 Y/O just got used to two persians, then about 2 months later another. Now they all coexist, although he doesn’t really like any of them, he doesn’t seem bothered that they are here either.

Also, when you get to the co-habitating part, make sure you have plenty of litter box, food, and water dish space for everyone to reach any time they may want. No fun dealing with turf wars over a litter box.