The gap does not matter. Actually, the gap might be the ventilation although I think there are vent slots on the back somewhere.
The probe slot is for your thermostat probe. You absolutely need a thermostat in order to keep temps from getting dangerously high. Reptile basics sells some good ones.
Have you thought of using a rack instead of display cages? If you plan on getting 6 snakes then those cages can become a pain to clean, not to mention they are huge.