I've never had problems fostering off from large litters. We had a female here birth her first litter last week of 18 and another female deliver 2 hours later of only 7, so we fostered off 5 to even up the numbers. Just try to foster between like size (age) litters if possible and mix the fosters right in with her own babies. Most female rats won't care and will immediately take over the care of the newcomers.