I looked into fostering a couple of months ago, they all recomended keeping the foster cats in the spare room to prevent spread of disease and illness and to prevent upsetting the ebb and flow of multi cat relationships.

If you still want to foster.. go for it. Keeping the cats to one room on a temporary basis will not hurt them and it is 100% better than living as a stray.

(PS. I decided against fostering.. I am too weak and would end up with 50 cats)