The backstory here is someone moved and left their 18 year old, deaf and almost completely blind cat behind, which my in-laws took in, but since they are both old and fragile, they can't really care for him besides providing food, a litterbox and a warm place to sleep for the time being.
We just took him to the vet for a complete check-up, and apart from a bit of weakness in his hind legs which the vet attributed to arthritis, he got a clean bill of health.
The vet says he can only see shadows and differentiate between light and dark at best, and does not seem to react to sounds at all, so we're working under the assumption that he's completely deaf.
Otherwise his body is still strong and muscular, he is eating well and using his litterbox properly, no indication that he's given up just yet or is about to anytime soon.
Now the problem, since my in-laws can't really take care of him and provide activity, if we can't find a better suited home which seems unlikely at the moment, we'll have to take him in and introduce him to our three younger cats.
Our youngest is a very social, and somewhat wild female just about a year old, and the older two are brother and sister just over two years old.
None of them showed aggression in the past, though the young one enjoys roughhousing with our tom quite a bit.
We're experienced in "normal" introductions, but I am somewhat worried about first direct contact after the initial exchange of smell and settling in, especially with such a huge age gap, and the fact that the old tom will not be able to tell the other cats approaching until they are very close.
Normally, we'd take a back seat and observe in that situation, only intervening if things start going poorly, but I'm wondering if physical assurance, i.e. petting the old one while the others approach him, would be the better route to take.
I'm specifically interested in tips and experiences regarding this special situation and the disabilities involved combined with the age gap in play here, that aren't covered in the typical cat introduction playbook.
We managed to find a suitable home for him in the end, and the introductions there went flawlessly. He bonded with a 17 year old female right away, and while a 6 year old male was suspicious at first, no problems did arise.
It appears my worries were somewhat unfounded, as him being unable to engage in a staring contest with the other male seemed to have avoided a lot of possible tension in the first place, allowing the other cat to observe without being challenged, or even noticed for that matter.
While the situation itself is resolved, I'll leave the question open in case someone might come up with a definitive answer that isn't situational and somewhat depending on the personalities of the involved cats.