1 inch per gallon is BS. It's a myth perpetuated by fish stores to sell more product. They know you will either buy fish for the 1 inch per gallon rule and have to keep buying them as they die off from water conditions or aggression, or you will be back for a new, larger tank set up. Either way they win.
You get a fish at 1 inch, like an oscar, once it reaches full growth at about 14inches (which sadly, because it's in a 10 it won't, but this is an example) it has obviously out grown the 1 inch per gallon rule. That's cutting the quality of life for the fish. Is that fair? That's akin to you living in a dog kennel. Not a fun thought right?
Bigger fish like those need the room to swim freely, hide, eat, etc without bumping against another fish. Eventually you will start noticing aggressive behavior because they are in close quarters.
We're in the process of setting up a new, larger tank to re-home our fish that will be out growing our 55g.