You're better off if you can use a thermostat. That way it compensates (somewhat) to changes in the room temperature. A rheostat is nothing more than a glorified dimmer and it'll crank out the set amount of juice whether it needs it or not (or needs more). If you're always around to tweak it, that's fine.
You don't really need an insulator between the heat mat and the plastic containers, so long as you're regulating the heat put out by the heat mats. They're only going to warm up to the mid 80's anyway, and thats well below the temps where you'll have issues with the plastic. If anything, with an insulator, you'd have to heat the mat to a higher temperature, which might cause issues with whatever the mat is on, but probably not. If you really want to even out the temps put out by the heat mat, you can get a very very thin sheet of aluminum at a hardware store and that'll do just fine.
Good luck!