I used to be a firm believer in dog food was for dogs and only dogs, and that rats should be fed something that was formulated specifically for them. I had always fed my feeder rodents <a href="http://www.mazuri.com">Mazuri 6F</a>, which I still consider to be an optimal diet and very affordable ($20/50lb), until the past few months. I have switched to using a low-quality dog food. My reasoning for this is that the nutritional information is similar to feeds formulated specifically for rodents, the higher protein content makes them grow faster and supports nursing/pregnant females, and the low-quality stuff is mainly vegetable matter anyways. The small amount of animal matter that is in there, is more beneficial in my mind, in terms of gutloading rodents for snakes, than a stomach full of purely vegetable matter - snakes aren't vegetarians I also give mine the occassional bit of veggies, fruit, etc. as a treat.
As for your question on hoppers, I am making my rat hoppers out of 1/2" hardware cloth, and my mouse hoppers out of 1/4" hardware cloth. I think 5/8" hardware cloth would be best for mice, but I cannot get any around here so I'm going with the 1/4", young mice can easily climb through the 1/2"