1. Get tubs with the locking handles. I think the 32 qt has neon green handles, but they may also have different colors. No escapes here.I personally just have a single sheet of newspaper over top of the 2 stand alone tubs that I have at this time. Both have fed very consistently and have shed perfectly.
2.Flexwatt...either wire it yourself or have Rich at RBI do it for you, either way, its cheaper than those UTH's...oh and make sure you have a thermostat that can handle whatever wattage you need. Rich can also help you with that.
3. Many racks have heat built in. It's also about insulation as an enclosed type rack, like RBI or Boaphile will help hold heat better than an open shelf.
4. I wouldn't stack the tubs, but mainly for a matter of having to unstack them all if a snake at the bottom had to be cleaned or fed.