That is good. once a month is just about right for rodents. I would rather work with rat pups due to the fact that they are easy prey. i mean hopper mice are really fast, they are faster then adult mice. I dont see anything wrong with a varity honestly. I don't think a rat pup once every other month would be bad. It is just when they are fed frequently problems arise. Nothing wrong with being too causious but i personnally would do it if it is more convienient and eaisier for your frog to catch. Then when the frog starts to get a little fat i would lay of rodents all together for a while. But as you already said you are feeding once a month at max. What you are doing is just fine. But like stated before a hopper mouse will have better nutrion.