Another thing (Which you'd never know when buying one small) is that not all animals are going to reach the "Max" size that that specific species can grow to. Are all Humans 5'10" tall? No, not even close but we are all the same species. Are all Chocolate Labs 75lbs? No but it's a rough average. Just like with fish, sometimes you get a freak that is huge and other times it's a runt. Just because someones fish does not grow 2" a month and end up 36" long doesn't neccessarily mean they are doing something wrong.
I mean go flip on the TV, I see people who are full grown adults who are <5ft tall as well as others playing in the NBA who are over 7ft tall.
Same concept if you ask me.
If all fish grew to maximum size everyone would be catching 20lb Bass and 40lb Northern Pikes, but they don't. In fact in Florida the average Large mouth Bass I would catch is much larger than here in Northern Indiana, but alas they are the exact same fish. Different conditions, temperatures, available food, and so on play a huge role and people forget that. A fish can be perfectly healthy and not be huge.
As to the original poster, you could surely get a small RTC or TSN for your 75g but odds are it would outgrow it pretty quickly. If it does and you can't get a bigger tank at the time sell it on here, larger ones pull a premium and they are a gorgeous fish so at least you'd be able to enjoy it for a year or so if you got a tiny one to start with.
The LFS by me had an adult RTC in a small pond in the store for a long time, not sure if it died or they sold it but it had plenty of grow space and wasn't anywhere near 5ft and I know it was there for many years. The pond from memory was around 6x4x2 or so give or take a little bit it's been a while, not a massive pond by any means but it had room to swim and grow with ease. If I had to guess it was 2.5ft at the most and likely weighed at least 10lbs, it was fat.