Pregnant moms can be quite easy. They, and the litter, should be segregated, either in a separate room or a large crate. There are volunteers in my organization that use both methods, and they work. I personally don't let them into the "general population" until they're weaned, wormed, had their first distemper, and tested neg. for FIV/FLV.

Good luck, and thanks for being bitten by the "fostering bug"!