Here's two great articles on AC that are devoted to stocking suggestions for smaller tanks. I really wish I'd received information like this when I was first starting out:
http://www.aquariacentral.com/forums/showthread.php?t=97203
http://www.aquariacentral.com/forums/showthread.php?t=120192
I think that making a "Suggested stocking list" for different size tanks would also be a fantastic idea. Sometimes even if people know better than to try to fit an oscar in a 10 gallon, they still don't know just how many neons or whatever they *can* fit in the tank, so providing them with a list of numbers of each fish that would be appropriate would be best.