And it doesn't end there: you've then got to consider if any of your proposed fish are fast swimmers (need a lot of space), heavy waste producers (need more gallons/inch or extra filtration), schoolers (need to be kept in groups with their own kind), territorial (must not be kept with their own kind), or fin-nippers (must not be kept with fish with long flowing fins).
I tend to go by the inch per gallon rather than the surface rule, as that allows less fish- I like to be lightly stocked. I like to be on the safe side should I miss a water change, and I like to feel they have plenty of room.
However, surface is important in that a high narrow tank (like a BiORb) will house fewer fish than a standard rectangular one.