Should i keep them separated?

So my grandmother passed away and when my parents go to her funeral in Tacoma they are bringing back her cat that she wants me to take care of. So she got this cat from someone who got it from a breeder and its a purebred Siamese. The Siamese was never named and never fixed, but now it is 16years old. I have a cat that hasn't been fixed yet and i do not want her to have babies so we are planning on getting her fixed soon.Should i separate the two until i get her fixed or is the boy to old to even try to mate? The last time i saw him he could barely walk. Also any tips on how to make him comfy. I have never had such an old pet before and this cat was my grandmother's favorite cat so he really means a lot to me. I don't want to stress him out or anything so does any one have experience on taking care of older animals?