The effects of kidney failure can be reduced, but not cured, by medication and a low-protein, low-sodium, low phosphorus diet, which produces fewer waste products. Potassium supplements prescribed by the vet may help too.
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These links have information and advice on introducing cats which you may find helpful.

http://www.messybeast.com/first-impressions.htm
http://www.littlebigcat.com/index.php?action=library&act=show&item=cattocatintroductions

Some cats are just more territorial than others. Our female cat chases off any other cat that tries to come onto our patio area. Whereas our male cat is much more sociable and he’s fine with other cats. If your cat is fairly laid back, you might not have any problems whatsoever with introducing them to a kitten.

Hope everything works out. Good luck.
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If your cat is not in final stages, this is not want you want to do. You do not want to put your cat on the
Vets low protein diet. There are simply so many better options out there than to hand the poor cat a low amount of poor quality protein – which often results in a poor appetite and muscle wasting since the body is now robbing its own muscle mass to feed itself a decent quality and amount of protein.
. There are new thoughts on this and the thinking is it is not the amount of protein but the quality of protein that matters.
The Merck veterinary manual [www.merckvetmanual.com] says that cats need "4 g of protein of high biologic value per kg body wt/day". That’s about 7 calories from protein per pound body weight per day. If a cat isn’t a good eater and consumes, say, 20 calories per pound per day, then 7/20 = 35% of calories can safely be from protein. It must be high quality protein, which means meat, fish, milk, and eggs, and not grain or soy.

I am under the assumption that you have been feeding mostly dry foods. Many use a vegetable based protein instead of animal and that is part of the problem.. Your cat needs protein as it is a carnivore and cutting down on it will lead to other health issues and may cause faster degeneration.
You want to cut down on fat and phosphorous (no fish allowed now) The best way to do this is with a raw diet which you can make yourself or buy. (Making yourself is better) link provided at the bottom
If you are unwilling to do that then something like the non fish flavors of Wellness or Merrick with NO grains are good alternatives. Wysong is also a good canned choice. This list gives a breakdown. Remember you want low phosphorous
http://webpages.charter.net/katkarma/canfood.htm

You also will want to look into phosphorous binders. Something like aluminum hydroxide
Ask your vet or look into calcitrol

You may want to talk to the vet about having injectable Pepcid AC on hand or you can buy it in pill form (ac not plain pepcid) and give 1/4 tab for stomach upset which happens a lot in crf cats due to acid in the stomach.
I hope this stuff helps, here are many links for you
Making cat food
http://www.catinfo.org/makingcatfood.htm
other links. Read, read, read!!!!
http://www.felineoutreach.org/EducationDetail.asp?cat=KidneyDisease
http://www.marvistavet.com/html/body_chronic_renal_failure.html
http://www.felinecrf.org/
http://www.felinecrf.com/
http://pets.groups.yahoo.com/group/Feline-CRF-Support/
About that vets diet. This report is for dogs but applies to cats
http://www.geocities.com/Vienna/Opera/2167/bovee_protein_RD.pdf
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