I think sometimes pups are sold off so that their siblings can get more milk and grow faster and stronger. Generally, rodents are culled for 2 reasons. They are runts or they don't have the color/coat that the breeder was looking for. As long as they are healthy, they should be OK. Rodents are very tough little critters. Don't let them breed too young, some of the mice people can help you with the timing better than I can.