That is great! It is best not to foster in the first 3 days if you can help it unless the foster mom also just gave birth. The first 3 days the mothers milk is Colostrum which is full of antibodies and if the newborns don't receive it by the foster mother then they are less likely to survive.

That being said I had a bad situation where a litter of 11 mother died when they were about 5 days old and I had to bottle feed them and foster them to a mother with a litter of 11. It was a lot of work. I had to check their weight and rotate them so that the smaller litter was getting enough milk without the older rats (older by 1 1/2 days) didn't always win the nipple.

To make it worse the second mother also died. AFter feeding off 2 of the weekest babies, I was down to 20 and had to foster them to a mother of 8. I also gave the whole litter formula with cooked oatmeal so that the older one were not as hungry, and the mother loved it too. So now I had one mother with 28 babies. I was so glad when they finally hit 21 days and I could ween them. I still gave them Formula mixed with oatmeal for about a week just to make sure they were fine. In the end it all worked out fine. Good luck!

Oh and Cdavidson I would separate them, their balls drop pretty early, and they can actually even get the mother pregnant again if they are left with her too long. You could probably leave the girls with mom though if you don't have enough separate tanks.