Mice are mice and rats are rats. Completely different species.

How many snakes do you have and how many mice are you buying per week? I use around 30-40 mice per week on average.

I use laboratory style cages to house my groups of mice. The sizes of groups range from 1.2-1.8. The size of the lab cages are around the 10-gallon tank size. Lab blocks are fed free choice(Mazuri Rodent 9F). They are put together and left completely alone except for cleaning once per week. I use kiln dried pine shavings(low dust) and they do alright on that. I don't mess with them at all during the week, no playing with babies, no separating anyone/adding anyone. They breed and produce great numbers like this.