I started with a mixed group of about 500 but it took several months to become established. Getting more than that will help get it established faster (but still think in terms of months and take into account how many you are feeding off). A heat mat at 90 should be OK as long as you allow ventilation underneath the bin. I'd also put the mat on a thermostat to prevent overheating. If you're worried about the plastic or can't put it on anything that would allow ventilation you could look into a ceramic heat emitter mounted above the tank on an adjustable stand.