I have a 2 foot tall cage for my gtp. I run a rhp which works great for him, but at 2 feet for a terrestrial snake the rhp would be nearly useless. 2 feet doesn't seem like much but it really is for a terrestrial setup.

I have three other pvc cages that are 15 inches running both rhp and belly heat. I would say 15-16 inches is the perfect size. They are not overly tall once stacked and offer plenty of room to clean and manage your snakes. Anything over that is going to be a waste of space stealing heat and humidity.

It sounds like your going with a one foot base and six feet of cage space. That's 72 inches of cage space. You could have three like you said but I think a better way would be four 15 inch cages and one 12 inch, or add three inches and have five 15 inch cages. You could also make the base three inches shorter and have five 15 inch cages at the same 7 foot height you planned for.