Flexwatt and herpstat question

I plan on buying flexwatt and a herpstat. I only have one rack but i plan on getting another soon. the rack i have is a 7 tub 41 qt rack or a 16qt 3 per level for a total of 21 rack. i was going to use 11in flex but that would be too much heat right? so i thought about the 3 or 4 but now im stuck this rack is going to be a hatchling rack until the snakes get older and need a bigger tub. so which tape should i buy. also im looking at the herpstat 4 mainly beacuse its features but say i bought it would it be able to controll tje diffrent flexwatts through that one console? sorry for such a long question just thought i should have a lot of the details.