To answer your other question, a rheostat is nothing more than a dimmer. Less electricity gets to the heating element - less heat produced. Very simple. The only problem with using them is that it requires more interaction by you. Here's what I mean...
Say on any given day, you carefully adjusted your rheostat to put out just enough heat to maintain your desired temperature of 84*F, as verified by your digital thermometer. That night, the temperature dips and the ambient temperature in the room falls 10 degrees. The rheostat will continue to feed the same amount of juice to the heater that it was before, and so the temperature in the viv will decline to some amount less than you wanted to maintain. The opposite holds true the next afternoon when it gets smokin hot outside and ambient temperature goes up. The temperature in the viv will rise accordingly, well above the temperature you want. YOU have to turn it down and up as needed to maintain the temperature you want. Essentually, YOU become the thermostat.
But, they are cheaper and relatively bulletproof, so as long as you are around to check temps and adjust as needed, they're perfectly fine. Personally, I use thermostats because our house is on a programmable thermostat and the settings go up and down during the day. This way, I know Vern's temps remain relatively constant.