Check out Animal Plastics, they have a good selection of smaller racks. They have models that are designed to switch between 15 and 32 quart tubs or thereabouts, exact size varies depending on brand. You probably won't be able to find a rack designed to switch between, say, 15, 32, and 41 quart tubs unless you build it yourself, though. As for heating, if you use Flexwatt it's pretty easy to wire the levels separately, then you can plug the boas' and pythons' levels into different thermostats to achieve different heat gradients. I'd probably keep them in separate racks, but I'm anal that way