The people who insist that a rheostat won't work after a thermostat are mistaken.
If that were the case, a thermostat could only be used with one particular type of heat tape. By adding the rheostat, as far as the thermostat is concerned, you're just changing the characteristics of the heat tape.
Say the heat tape would normally get to 120F with the thermostat at 100%. You could set up initially without the thermostat and adjust the rheostat so the tape is 95F when the room is at the coldest. Then you add the thermostat before the rheostat and set it to the desired temp. If the thermostat fails 100% on, your heat tape won't be able to go over 95F. If it fails 100% off, your snakes will drop to room temp, but that's a lot better than cooking them.

The better solution is to use 4" flexwatt, which is designed to run cooler, to avoid the whole problem. I wouldn't use anything but a herpstat to regulate the temps.