Fostering is different at the Humane Society than a 'rescue'...or at least, that's how it was here when we started doing it in the '90's. I am not sure how prevalent 'rescues' were at the time, though.
Fostering doesn't have to mean you take every baby offered. It does mean you can choose to take one too young for adoption. At a point, we couldn't take more fosters (babies)...our decision...because we had our hands...and house...full with babies of local ferals. But, the babies we took in to foster, only went back so we could sign the papers to adopt. We are horrible at giving up a fur baby!
Good luck...I can hardly wait to hear how it turns out.