I can't argue that most shoaling fish, never mind schooling fish, should be kept in the largest possible groups.
But that being said, most folks don't have tanks big enough to keep hundreds of fish, even if they are very small. One reason I love my Strawberry boraras, fifteen of them hardly take up a corner of a 10 G tank !
I'd like to have three dozen of the purple harlequins myself, but I don't have tank space for that many, even when the tanks are here.
[ because my landlord doesn't want to do repairs.. dragging his butt and making my life miserable as a result]. So I have only the small tanks on every available surface in my bedroom with the species that are less forgiving of errors and my friend takes care of the rest of them.
In a 10G, you don't have a ton of space for anything really. But once we start keeping fish in glass boxes, we fundamentally change everything about their environment anyway. Sites like Seriouslyfish, which I like very much, often tell you that species such as, for an example, kuhli loaches, should not be kept in groups of less than six. In the wild, they're likely congregating in the hundreds too, but that's not possible in most home tanks.
So I think the best that can be hoped for, is to do the best you can and have as many as you have space for, with a substrate that will at least not harm them and furnishings that are something like what they had in nature if possible.
If fish come from heavily planted areas, like Bettas do.. keep them with plants. I have so many plants in with my Bettas, I don't even need a filter on their tanks, the plants, and whatever BB live on the surfaces, do all the work. Testing has proved it over and again, the results are 0, 0 and <10, always. Don't even have to change water often at all, certainly not every few days. But I have to keep suitable lighting over them to grow those plants.
And hopefully you do research, as the OP here has done, to find out what possible species really won't fare well in the conditions you can offer and then don't get those at least.
It can be hard sometimes, to offer advice, when someone asks, can I have this ?, and you know the answer should be no. But saying no over and over isn't any easier than hearing or reading it over and over is.
We have a cute little triple log with fake moss on it, the catfish like hanging out in there lately since Miss Betta was moved.
