er, you have a point except that a 1-inch neon isn't even 1/6 the volume of a 1-cm puffer. the inch per gallon rule mostly applies to slim fish.
as for 300 cardinals in a 4ft, there was another thread today about tank sizes that mentioned a 4x2x2 or 120 USG tank. just remember, length is not the ultimate descriptor of volume.
someone else on the forum once mentioned this explanition for recommending the "1 inch per gallon" rule: if you have to ask, then you don't know. meaning that a person asking am i overstocked probably doesn't have the knowledge base to "maximize" their aquarium. and its a lot easier to just add up the inches than start analyzing filtration, substrate, volumetric bio-loads, heavy vs moderate planting, territories, and so on... and the strict maximum number of fish that can live in a tank is not the same as the number of fish that can thrive in a tank. why overcrowd if you don't have to?
....contrary to popular opinion, clean and clear water is NOT the best way to tell if your fish are overcrowded. no fighting, characteristic behavior, good colors, tyoical growth rate, common sense--those are a few other things that should drive stocking.
benny, the only thing i'd add is another cory or two. throw in a couple of plants to offset things, and it should be managable.