Try aspen substrate and see if that helps you with the humidity.
Yes, you can plug in as many UTH's in a thermostat below the thermostat's max wattage capacity. But, the thermostat only comes with one probe, so it only monitors one heat source and adjusts all heat sources plugged to it. So, it works best if all the UTHs have the same wattage and all your enclosures attached to the thermostat is of the same size/type/set-up. Then your heat variation between enclosures is not too different.