I think the better guide line is if it looks crowed your overstock
Problem with that one is what looks crowded to you might not look crowded to me. Most newbies (and they are the ones that need a rule to hold onto) have only got the lfs tanks to compare with, and they are overcrowded to an extent that you simply cannot reproduce at home without disaster. To most newcomers to the hobby, a healthily stocked tank will look incredibly sparsely populated. By the time you have trained your eye so that you see that an overcrowded tank IS overcrowded, you can let go of the rules anyway, because you will know quite a bit about stocking.
I think it's better for people starting out in the hobby to think of the inch/US gallon rule as the first in a set of about 10 rules, all of which are important. Some of the others go like this:
1. The inch rule only applies to slimbodied fish under 3 inches or so. No 10 inch oscars in a 10 gallon! And it applies to the adult size of fish. They need room to grow- before they have grown!
2. It does not apply to heavy waste producers (plecos need separate attention- and big tanks).
3. Schooling fish need to school. If you haven't got room for a school, don't keep those fish.
4. Fast swimmers need long tanks. Swordtails and danios should not go in a 10 gallon, where they can't stretch their fins.
5. Fast active fish should not go in with very timid fish. Some very timid fish may be outcompeted for food, and you will either need to keep them in a species tank, or target food at them.
6. Fin-nipping fish (serpas, lemon tetras) should not be kept with fish with long flowing fins.
7. Some fish are territorial. Either all the time or when spawning. The tank needs to be big enough for their territories. Some territorial fish need to be kept on their own, or without fish of their own kind. Two male bettas in the same tank can spell disaster. Two red-finned black sharks is another one that often gives rise to bullying.
8. Some fish require special gender ratios. For livebearers you need to either keep single sex communities or at least 2-3 females per male. For kribs on the other hand a trio can spell trouble. Research is the answer.
9. Some fish have special water requirements, which may not suit the water requirements of the rest of the community. Neons and African cichlids are not going to be happy in the same tank (quite apart from the bullying/eating).
10. Some fish are predators. People who keep black ghost knifefish with a small community of tetras may find themselves mysteriously short of tetras after a while.
11. Think of the tank as a series of stories. It is possible to be overcrowded even in a large house if everybody insists on living in the top story. A tank can be fine from the filtration point of view, but still overcrowded at the top, if you only keep surface dwellers. So all fish need to be researched for how they live and feed.
These are the ones I can think of at the moment. But the one key to successful stocking is research, research, research. Never buy anything unless you know what its habits are, what its needs are, how big it will grow and what it is likely (the best anyone can say) to be like with your other fish.
