Ball pythons can be big time imprint feeders. If you offer another mouse now, the snake may only eat mice for the rest of it's life. If that happens, it may be very very hard to get your snake to switch back over. I would stick with the rats.

Also I've noticed when my BP's "stalk" their pray they normally don't eat that time. They would rather harass the prey item then eat it. Also they are ambush predators. They usually use a hide, to hide in out of sight.