I am fairly new to fostering. Each shelter will have its own set of rules and requirements. Some pay for everything, some do not. Some require complete separation of foster cats from household cats, some do not. Those types of questions need to be asked of the group(s) you are thinking of helping.
Now, even if the group you choose allows intermingling of cats, they should be separate at first. (Just like a newcomer to your own cat family). This gives them a time to feel comfortable in their new environment and to clear them of any illness (including the common URI, which will spread fast through your own kitties if they are exposed)
After that, use the amount of time before inrtoduction based on how quickly the foster cat adjusts. A lot of cats in need of fostering have been trapped as strays and will not be ready to mingle as soon as some.
In the example of a mom cat with kittens, I kept them separate the entire 8 weeks from my own cats to help the mom cat to not feel threatened. When her babies got adopted and she was left, I allowed her to mingle with mine. Right now I have 3 unrelated 10 week olds and they have a room to themselves. the 2 more outgoing ones will be ready to mingle soon, but the scared one should not have freerun of the place yet because she would be impossible to find.
These are some exampls, I hope they helped some.