Silver bodied tetras are more likely to shoal than other colours. They use sunlight and the surface of the water as camouflage. I've seen rummy nose species in underwater video, and they seem to cruise in groups of maybe 50-100, not in massive shoals. In the vids, shot by a friend, they are upper midwater, but look like they run to the surface if there's a threat. In the dappled sun, silver is hard to see.
Shoaling is a means of confusing predators. Have you ever tried to net a fish, and then had them get away because others appeared and made you hesitate for a split second? Every pet shop worker knows how shoalers confuse the eye.
In those same wild underwater vids though, many silvery tetras seem to mill about in enormous numbers, not shoaling or schooling but making getting eaten less likely because of their sheer numbers. One fish can be tracked and predated. 2000 are like a buffet with everything you could want - hard to make a split second decision on.
The smaller, blunt shaped tetras like glowlights or neons have colour, and their world is the bottom half of the tank. Their safety is plants and wood - traditional hiding places. They will spread out like a herd of mammals, and forage while keeping an eye on each other.
A variation is the high bodied serpae type - not built for current running, and very set on holding turf along the bottom in loose, herdlike groups.
So look to body shape and body colour. If it is dark, it likes the shade lower down. If it is silver and built like a torpedo, it will shoal a little more. We're spoiled by documentaries on marine schooling fish, and we want that in our tanks. We won't get it. Maybe if you have an enormous 1000 gallon tank, you could get some tetras to shoal, if you had some fish that acted predatory too.
I just looked out my early morning front window, and a group of the ever present whitetail deer we are surrounded by passed in clouds of their own breath, moving through the trees in front of my house in the exact formation my cardinals will move through the Vallisneria in. I may want to see 3000 caribou moving like they do in documentaries, but my local hoofed ungulates have no reason to shoal.