Aspen, Carefresh, and anything made from recycled newspaper are safe. You should shred up some tissue or paper towel for them to use for nesting. Commercial seed diets that are sold in most pet stores are not sufficient diets. Lab blocks or pellets are much healthier. You can supplement your mouse's diet with small amounts of vegetables and fruit, seeds, whole wheat breads and pastas, cheese and other treats.
Female mice rely on the companionship of other female mice.
If you are getting female mice, it is important to have at least two. Male mice can be housed alone but often to find in pairs or groups as well. Visit the RMCA web site.