The inch per gallon thing should never have been spoken aloud by the first person who thought it up. It's a ridiculous concept (but you couldn't have known that). If anything, it only works for really tiny fish, like nano fish.
That said, when stocking, there are things you have to consider that are equally important to species size. Some fish require lots of room, like the sharks. They need to cruise the tank. Other species have a large bioload, and produce a huge amount of waste. You need to have a larger tank and hard core filtration. Some fish require schools, like tetras, who should be kept in groups of 8 or more. Some fish can be aggressive in a community setting, like the gouramis, and so they need more room to stretch their fins and claim their territory.
I watched my shark go through the fastest growth spurt I've ever seen. He then proceeded to eat my school of danios. He was telling me he didn't have enough space and I rehomed him.
If you're getting a larger tank, consider a 75 gallon if you're keeping him.
The best thing you can do is learn from our experience and our mistakes. Each generation of fishkeepers should have less struggles than the one before.
