2 kittens + 1 elderly--advice?

[GBP]

I'm waiting for the "multi cat kit" to come out soon (site says end of October). I've got a 4 month old ragdoll and a 7 month old ragdoll plus a 14 year old cat in poor health--lung tumor. I want the kittens to learn to use the toilet but I don't want to force my old guy to do it if he doesn't want to. So I really can't take away every litter box. Right now they have litter boxes in 2 rooms and all use them both, so I can't see where adding a box in a third room (bathroom) would be a problem for them to figure out. They are all really smart kitties. There was never a litter box upstairs in my old cat's life, and now there is, and he uses it without any prompting (I made a room for the kittens when I first got them, in case I had to confine them).
I also only have 1 toilet, so getting 2 litter kwitters isn't an option.
I don't want to wait until my old guy dies to train the kittens. (He was given 3 months to live in April and he's still around, going strong.) But I also don't want to spend $90 on this system and find out it won't work within the confines of what I need to support two very different types of cats.
Any advice?