Jenny,
My advice would be to let her acclimate a little longer, at least another three days. As for your setup, 4" flexwatt on a 12 qt. may not leave much room for a cool side. What are your temps on the hot side? My thoughts are they may be too hot, explaining why your baby is always on the cool side. As for the prey items, the breeder may have meant that her first meal was a hopper mouse. Many times, alot of my hatchlings won't take rats at the beginning, but have to be converted at a later date. I would get her a hopper mouse, and my gut feeling would be that she will take it. If she eats the hopper mouse, then offer her a rat next time, but be prepared to offer another hopper mouse if she doesn't take the rat. Then, every consecutive feeding, offer a rat pup first until she converts to rats, if that is your ultimate goal.

Hope that helps,