No Rheostat. Thermostats are what you need.

Those range from the cheap (but still works well) ZooMed ReptiTemp 500R dial style, to the good Johnson and Ranco on/off style, to the best Herpstat and Helix proportional style. If you can afford it, go with the Herpstat - found at Spider Robotics. That's the really really good one.

Oh, and the only way something would catch fire is if there were sparks, which could come from anything... it would take a LOT of heat to set a cage aflame - more than any pad can put out.