you can feed the raw shrimp their whole life, but if you want to do pellets, you probably won't be able to get them to eat them right away when they are small, so usually people wait till they are about three inches or so, that's usually when they stop eating each other on a regular basis, so it's ok to skip a feeding or two to make the pellets look more appetizing.
you said minnesota winter but your location is illinois, did you move to minnesota?