You know mice are fickle sometimes it is the case and sometimes the don't care. To be limit as much risk as possible I would get all the females and 1 male at the same time, make sure they are the same size and if possible get them from the same place.
Yup, I agree with this. A LOT of people have trouble getting started with mice. But once you can get them to breed and have a litter, start your new colonies from your own young mice. They will get along with each other and breed much better. I take all of my weaned mice and seperate them by sexes in grow out tubs. Then when I set up new colonies I take all of the females that I want for the new colony from the same grow out tub and select a young good sized male from one of the male tubs. I put them all together in a different freshly cleaned tub and they usually get along real well and I can expect a nice big pile of pinkies within the month. Often, all the new females will give birth within a few days of each other. If you start with older animals from different sources it can be pretty much hit or miss.

Mark