I think what you described are good measures to implement. I kept the fosters in a bedroom separate from my own cat, and my cat did not get sick from either set of fosters. I think it must have been very good luck or? that only one kitten of your set got ill. The last set I took was a mother...
Are shelters incapable of preventing the spread of URIs to their cats and kittens designated for foster care? I tried fostering but now have twice received a cat and kittens very sick from upper respiratory infections. The shelter seems to take it for granted that any animals in their care are...