Thinking about getting a rack

Hey!

I'm a new reptile/snake lover and am thinking of getting a rack in the next few months or so. I want to get one that has got built in heat already as I don't want to fuss with installing heat tape. I will use one thermostat for the rack so I would like to know what species can you house in the same rack that all have similar heat requirements. I like corns but also like kings and milks. I don't think I can have BPs with the same thermostat but I'm mostly interested in Colubrids.
I would get the racks that are full size and use separators or hides for juvenile snakes.
Also I see on the racks they ask if you want the standard back heat or belly. I don't understand why people would use back heat. Heat rises and seems like it would not do what it supposed to do. May back heat is used for arboreals? Even so it seems highly inefficient.
Thanks for reading guys.