I use Herpstat 4's. I have 7 snakes an 2 lizards.
I use PVC cages so each cage gets it's own Channel on a Herpstat 4.
For a rack
I would get the Herpstat 1.
I would get flexwatt prewired from reptile basics
http://www.reptilebasics.com/flexwatt-heat-tape
32 quart tubs will be fine for now, if any of your snakes get really big you might need 41 quart tubs.
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The Herpstat intro isn't the best choice for a rack. They are geared towards incubators.