The one inch per gallon rule is a farce. Think about it, going by that a 10 inch oscar would be fine in a 10g tank. What they don't explain with the one inch to gallon thing is that you need to consider the adult size of the fish.
I go by my own rule, I know how big the fish should get by researching, then I mentally "divide" my tank into sections, from that I can say this fish needs roughly this much room. Then I go by what parts of the tank they prefer. Like my dragon gobys in our BW tank, they are bottom dwellers. The glass fish in there are mid to top, the mollies are all over and same with the shrimp.
But yes, bio-load also plays a big part in tanks. Some would consider our 115g tetra/angelfish tank grossly over stocked: 120 neons, 9 black neons, 5 harlequin rasboras, 6 glow light tetras, 12 guppies (for live food) 4 angelfish, 4 GBRs, 1 chocolate/albino pleco, 12 ghost shrimp, 9 nerite snails, 6 mystery snails, 3 peppered corys, 3 albino corys (with 10 more in QT). Sounds like a lot of poop right? That tank gets cleaned every other week and has less waste than our 115g or 120g cichlid tanks. Those get cleaned every week.