Well, with fostering a pregnant mom, mom does most of the work. But you do need to be prepared just in case mom rejects the babies (I never had this happen to me, but it does happen).

I never covered the crack under the door, but it would depend on each individual cat. Some might need that total separation, and others won't mind a paw or two under the door.

As far as keeping them in one room, start by asking the agency you end up fostering for. Some have strict rules on "mingling".