new cat intros

How long should I separate my existing 8 month old cats from a new pregnant mom I'm going to be fostering?

I am getting my first "official" group later today and my shelter is adamant that the cats should be kept separate due to unforeseen health issues that could be lurking under the surface. Plus you don't want to stress out a pregnant mother. Queens are very protective of their babies and they need peace and quiet.
My house cats have never met my foster kittens(I decided to foster them on my own) who have been here since birth 7 weeks ago. I don't think it is necessary. Only one who is the runt is ever around them. They know that they are here as I take them out and play in my TV room but I close the door to keep my cats out.
In my mind it is easier because it cuts down on territorial issues and unnecessary squabbles. If it were 2 orphans I would let them play with the house cats to learn valuable socialization which would make them easier to adopt in the future. Cats that like other cats get adopted faster.
But a pregnant Queen needs her own space.

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.