i know that 1" per gallon rule is ... it's a good place to start, and it takes almost nothing into effect
goldfish & the common pleco are notorious for eating & pooping lots, this pushes the bioload far higher than a standard fish that has that typical narrow fish look to it.
fancy goldfish have all the mass squished into a short body, discus, angelfish, others, have all the body size, but a shorter body, ... fish that eat & poop lots, doesn't matter their size, they eat & poop far more than the average fish...
dietary requirements rise just for larger fish, again throwing this off, ...
your right, the 1" per gallon rule of thumb is ... its' a good place to start when you don't know much about how to stock a tank, ... then they start rating fish on bioload ... how much will that one fish increase ammonia (and ultimately nitrate) levels in a tank due to their natural body functions
within reason, i have decided (just my own arbitrary decisions) that minimum size aquariums are about 10 gallons for every inch of fish, ... just to ensure the fish has enough room to swim in.
it's not always going to be right, some fish are fine moving slowly, so a smaller tank is fine, ... some fish really want & need a larger tank just to have room to really get going, ... but it's a good place to start i think if you don't know anything about the fish.
in both cases, always best to read up on the fish in your own personal time instead of looking for what others say, if unsure, err on the side of caution and get a larger tank.
best advice i ever heard "get a tank large enough for your fish when they have reached full grown size, because every intent of "i'll get a larger tank when they need it", ... your fish will continue growing till it's cramped and unhealthy, and that will continue till it dies, stressed & unhappy, ... the concern here is you have every best intent at heart, you can look at your savings and know "yes, this is reasonable, i'll be able to get a larger tank when i need one", ... but how sure are you that when you have to get a larger tank there hasn't been some emergency that was more important that spent that cash for you, ... vehicle repairs, family emergency, something came up you were not ready for.
getting a tank that is minimum size is ... never a good idea.
stocking a tank to the max is never a good idea.
and these are moderately fine with community & friendly fish.
if you've got aggressive fish in the mix, ... the smallest tank size you will ever get is for a breeding pair, sure they're still territorial, but like any couple, you make exceptions for the one you love, you do want them closer and that helps, ... in that case you can put 2 oscars in a min. sized tank for 2 oscars. if they're a breeding pair. ... i wouldn't recommend doing this with 2 males, ... and 2 females are questionable.
if you've got a 75 gallon tank and want to push the limits, sure get two oscars, ... learn to sex them so you can tell ASAP if you've got a male & female, ... if not, swap out one of them and get a different one.
and i have zero knowledge or advice on how these fish treat their young, ... maybe their food, maybe not, ... but gotta look into this stuff.
less pride, less ego, less stubbornness, ... it doesn't matter what you want, if you push ahead in the direction it sounds like your going, your going to be really upset to find one dead. and then is the realization that it wasn't worth pushing the limits, ...
do your homework, do a ton of research, put aside your ego, these aren't lego that are yours to play with, they're living creatures that have their own quirks and likes, ... you can want whatever you want but if they want different you're going to loose, ... and possibly the fish will loose too (big time)