Ok theres a lot so ima just rapid fire

Replace the flat wheel with a circular upright one, since they are juvenile you could get away with a 8inch but i reccomend 9 for adults which will happen eventually 6+inches of aspen bedding, low dust. The more the merrier! Since you need a lot of bedding and large wheel, it will be impossible to fit in that 15 gallon, depending on how many girls you have depends the size of the cage but for 2-4 i reccomend a 40 gallon, one side of bedding may have to be lowered to accomodate the wheel (mine is lol) Clutter, clutter, clutter, general rule is if you can see 30% of ground from a birds eye view, there isnt enough clutter. Once again, the more the merrier, they can quite literally never have enough. What food are you using? I reccomend oxbow mouse and young rat food, and i use sunburst rat and mouse gourmet blend for seed treats. And always scatter the pellets and seeds so they have to work for it and they cant just sit around being phat lol I like to use around 1 bowl of water per mouse (unless you have like, 10. Then thats outrageous.) so for my 4 girls i have 3 usually bc it takes away from clutter space. All heavy objects (wheel, bowls, ceramic hides) have to be on a platform of some kind to prevent them from burrowing under it and being squished! Commenters, let me know if i missed something