Evening Esther. I would go:-
All Corns together in the large wooden viv.
Two larger Garters in the large plasic.
Baby Garters together in the small plastic.
No, I wouldn't...I'd sell the Corns!

I always use heat cable with my slate floors, it's fine as long as you use a thermostat of course. It is best to position the cable to one end of the viv, and so create a thermal gradient (warm end, cool end).