katladyd, your situation sounds a lot like mine, I have limited space here (enough for two cats, though) and one of my cats has problems with struvite crystals and bladder infections; and she tends to pee outside her box when the stress level is too high. So I can clearly see the dilemma you're in.

It's true that we cannot save them all, but the fact that you are already in love with the kitty from the shelter doesn't make it easier for you. Maybe you can take him home with you for a couple of hours to see how your kitties react to him? If that does not work, I think I wouldn't put the stress on the resident cats.

Kirsten