It's not about the hot spot it's about the ambient temps, you can use a regular 3 to 4 inches strip of heat tape but you will require a room heated with a oil filled heater and a reliable external thermostat set to at least 75
As for mice you build a rack with plumbing system and you set up several harems and have several tubs for weans and grow out.
You need to start with small weaned mice not adult raise them up to optimal size and start pairing them when ready.
Number of mice you will produce will depend on how many animals you will produce.
I'm looking at getting one breeding pair (To make Banana Clowns/Banana Pieds) so I wouldnt have very many offspring, 8-9 hatchlings probably. I actually didnt think about an oil filled heater, it would probably be my best bet.