I would encourage you to dump the walmart rodent food (pet store food pretty have zero nutritional value)

See if you can provide them with Mazuri or similar lab block designed for feeders.

As far as separating it can be done, some people do harem some, run a male through the tubs where females are located. It's really a matter of preference.

Personally I like to run 1.2 or 1.3.

Make sure to provide the right food and make sure they are not overcrowded, then see how it goes.

If it does not work start from scratch with new breeders.

Mice breeding can be more tricky and it can take a few attempts, took me to attempt to build a solid colony that produce well and that does not cannibalize their youngs.