Yeah, many fish should generally be kept in groups. Botia loaches, Cory cats, almost any schooling or shoaling fish (tetras, barbs, etc.), lots of other stuff. And some stuff should almost always be kept singly--puffers, eels, etc.
Ok the question is why we buy fish in groups like trio or pair. There are 2 main reasons:
1. Fish will feel better in groups or school. Some fish like to school. Most juveniles or babies will school until they reach maturity and then some species can go solitary. So u should buy at least 2 or 3 same type/species of fish whenever your tank permit. Of course if u dont have space in your tank u shldnt buy at all. There re fish that reaaaaaally like to school like the silver dollars, u should buy at least 5 of them.
2. Some people buy fish to breed them or at least hoping or planning to breed em.
You are half right . A schooling fish would still grow healthly alone if fed well. But keeping it in a school promotes better growth in their mental health. Which means they would feel much more happy and comfortable in schools. Allows them to interact and form social bonds with other fishes in the school.
It does too...the reason is that they eat more aggresive because of competition. I used to own 1 oscar and when i added another 1 he became more aggresive in grabing the food
i have a couple big community tanks and i house lots of agressive fish similar to yours. the worst part of it all ive noticed is having to catch that 1 fish thats being an sob. 1 net 2 nets, a divider, 3 nets, damn you.......
good luck i like the grammodes the best on your list!