Oh, how exciting! I love baby mice! Like Vanilla Rat said, they should be fine. I was in a similar situation with my two litters. One mother had had her babies at the store but I was using her as a foster mother for part of a litter, and a couple days later a mouse I'd bought at the same time as the foster mom had babies too. They shared the work but for the most part mainly took care of their own babies. I had no problems with them at all, even though the foster babies of the first mother were a week and a half older than the second set of babies. (The little ones would cuddle with the bigger ones, it was so cute).