To reply on another mouse raising the babies....
A couple weeks ago, I had one female have babies(1st litter), she has a very helpful female friend in the cage that is extremely helpful.
Anyways, 2 days afterwards, I had another female (In a different cage) have her babies(2nd litter, 1st litter, none survived), I caught her cage mate (another pregnant female, 1st time prego) about to eat one of her babies. I did save the baby before she could kill it.
The 1st mom and her cage mate were doing very well, so I was hoping that if I put her and her babies in with the others, they would all 3 (the 2 moms and the very helpful one) help each other take care of the babies.
I put the babies in the nest, under the older ones. Then introduced the 2nd mom. The 3 females got along okay (they gad been together before). However, the introduced mom went into the nest and severely injured 2 of the older babies, enough that they did not survive.
I was mad at the little brat.... not going into details, but she ended up being fed to my ferret.
So, there ended up being 6 babies that were about 4-5 days old and 5 that were about 3-4 days old. The mom and her helpful friend completely adopted the introduced babies and have had no more die.