I am also a first time mouse breeder, about 4 months now I have left my 1.2 trio in a 10 gallon tank during and after birth. I just keep them all together and do "mix" them up after I setup the colony.

On a side note.... I have had two litters born about 7 days apart. They had the babies in the same nest and often share feeding responsibilities. I have noticed that the younger pinks struggle to nurse as the older ones seem to "muscle" their way to the nipple.

I keep them in the house so...room temps