2 + 2 + FIP = crisis

I recently moved myself and my two shelter cats (Ruby, who's been with me 11 years, and Lucy, 8 years) into another household with two rescued cats of its own (Gizmo and Nuke, both about a year old). The strangers are still segregated from each other, with the newcomers confined to a previously cat-free zone. Trouble is, Nuke has only been in the household about 3 months now and was quite thin when adopted. He's been acting a little off lately, a little slugglish, and after a few weeks of trips to the vet, no response to antibiotics and treatments, he abruptly declined over the past weekend. I can't believe how quickly he just started to waste away. Febrile, not eating, completely lethargic. 2 days ago, after a delay to get bloodwork results, the vet dropped the bomb that she believes it's FIP and gave him the usual poor prognosis. 2 days in, we're seeing all the improvement on prednisolone that would be expected, he's eating, has more energy and acts like a cat again. Bully for that, he is comfortable, and we will do whatever we can for him, for better or for worse. In the meantime, I'm having a total crisis about what to do about the 2 cats I moved in. How do I provide for all four of these cats at the same time? It's more or less a certainty that both of my shelter cats have already come in contact with coronavirus from their year-long residency at the shelter with numerous other strays, and of course Gizmo is in that same boat and is also apparently healthy as an ox. I can keep Ruby and Lucy separate indefinitely, but not permanently. I have a friend who is willing to foster my cats temporarily in her intentionally animal-free household, but can we even safely integrate my 2 cats with Gizmo later, after a thorough housecleaning, if Nuke follows the expected course? I'm really not sure what I should be planning for. I was so looking to having a big happy family and would appreciate whatever guidance can be given to keep these kitties safe and healthy.