I haven't used that bran dof heat mat, but I don't see any reason why it wouldn't be OK. Just make sure it is only underneath about a third of the tub/tubs. Did you know you needa thermostat as well?

You can use aspen bedding, but personally I use kitchen roll for hachtlings. It make sit far easier to feed them without having to worry about them eating bedding etc, and is easy to keep clean and hygenic.

Temps need to be around 30 hot end, 22 cool end. Don't worry too much about humidity. it shoul dbe about 50%, but if you have a water bowl int he cool end, it'll be about right. if you notice any problems shedding, move the water bowl nearer the heat course.