The 1" per gallon guide is a good starting guide to allow them to ease into the hobby without many problems.
im actually starting to think that inch per gallon rule is bad. once you start working at a fish store, you will see why. i get people asking me if they could get 2 koi in a 10 gallon tank because they are 5 inches, and the tank is 10 gallons. horrible rule if you ask me, but then again, i get all these wacky questions anyways like, can i keep that fish in this bowl? (bowl is about the size of the cup and the fish was a domino damsel) i think when you say an inch per gallon, it gets stuck in their head thinking that everything is okay as long as it fits that rule. i think a better rule would be, if you wouldnt want to be that fish, dont get it.