Pregnant moms usually do not get vaccinated and generally have not been with the shelter group long to watch for underlying medical problems etc so I would keep them completely separate if possible for the whole time you are fostering. It is part of our foster contract for pregnant cats that they are not introduced to resident cats.
My foster was a feral and up to date on all shots and had been at the shelter for quite a while so it wasn't a huge issue to me when she broke out of the room and my cats had survived the hour or so she was out. She was always going to be a possible adoption anyway so I was introducing her to them at some point.
Pregnant cats or new momma cats can be vicious in protecting themselves and their young too, no matter how friendly they usually are. It is also not a good idea to stress them out in general.