Sharing the water fountain is no issue however the new cat may not like it. We have to have a running water fountain for one of our cats while the other prefers a still water dish that I clean every day. Not sure why but she just never liked moving water. I have always been told by my vet that you need at least 2 litter boxes if you have 2 cats. However it probably entirely depends on the cats involved.
I guess with the water it will be a wait and see approach for her--however I'm glad I didn't throw out the previous water dishes/litterbox, as they will def come in handy.
My two cats share a litter box, but they were from the same, uh, litter. Born in a group of five, and we kept two. Results may vary, of course.
The foster baby kitties we have currently are kept completely separate from our two cats. Your new one is more than likely already vaccinated, so you probably don't have to do that. One of the kitties faded :(
Vet detected pan leuk in their stool so we had to do a thorough sanitization of the area. The other babies are doing great, thank goodness.
I am very sorry for your loss--pan leuk fucking sucks. Happy to hear the others are alright as of now.