I agree 100% with LBF.
A fish can't put it's tail between it's legs like a dog can, many people say their fish are "happy" in substandard conditions because you can't read fish very easily. . . Also, a fish will adapt over time to bad conditions, but that doesn't mean it is right. People can adapt to malnutrition and somehow survive, but that doesn't mean that they should be kept that way because they've already adapted to it. I've seen people claim every shoaling fish imaginable is fine on it's own. . . That's why it is important to look at the natural behavior of the fish as an example, and if in the wild this fish makes a free choice (instinctual, whatever you want to call it) to live in groups with its own kind, then it should be allowed to do so in our tanks.
As hobbyists our goals are to raise the bar, keeping aiming for better. 20 years ago, 3 tetra would have been an acceptable shoal size. These days, most will recommend no less than 6 of a shoaling species, and more is always better.
I've never kept these fish, but LBF says they are active so I will go by that. . . Choosing fish for your tank isn't an exact science, more about how you feel about crowding a fish up. None of the fish I keep in my smaller tanks are active, they are either slow swimmers or move in very short dashes at best. Fish that like to live the fast life deserve more space, in my opinion.
That said, there are a few shoaling fish that could live reasonably well in a 5 gallon or so tank, though none that I would risk with a betta.
The reason these fish are famous for dying off, is that in the wild many of them are fished using cyanide, which causes fish to all rush to the surface gasping and makes them easy to scoop up. This damages the fish internally, where it may take a while before you notice. As far as I'm aware this practice is becoming less common, and otos may be bred on fish farms now, so incidents seem to be decreasing.