We have three litterboxes. Two very large tupperware containers for the three younger cats (scooped daily) and one box for Beeper.
Beeper and the younger cats do not get along (well, I'm sure they'd
be fine, but she's a snit
) She was a feral when we found her, and got into more than a few cat fights in her younger years. I think that soured her to all future cat/cat interactions.
Luckily, we have the space so that there's really no detriment to keeping them seperated. I don't even think they notice most of the time
Because Beeper won't be around for too much longer, and I'll be moving out with the younger cats in a couple years anyway, we can afford the extra work. I'm not sure what we'd do if it were a more longterm situation...
With your kitty being sick, I would hold off on getting a new kitten or cat. It will be a bit of extra work, and it would probably be best at this point if you just focussed on caring for your girl. Not to mention, a new cat could stress your kitty out even more, not something you want to do if she's already ill.
If she gets better, then I would definately consider it. And if, God forbid, she doesn't, then you may want to take some time before opening your home to a new cat.