If you put the reflective tape under the flexwatt and then the flexwatt I think it would work. You going to use sweater boxes or using tanks? If your using sweater boxes you might want to find away to add more shelves.
The size tubs would depend what you want to put in the tubs. For hatchlings the 6 quart tubs, adults 12 quarts-16 quarts, for 2-3 then use 32 quart tubs.
I would use that reflective tape under the flexwatt to try to keep the plastic from melting. Since you are going to add 2 more shelves then get the 5 shelf unit so you will have 7 shelves to use. Let me know how it works for you. I just might do the same thing.