Don't get me wrong - Cardinals are among my favorite Tetras. Their coloring is spectacular, but as is the case with many smallish tetras, their schooling is weak. Once settled in and acclimated, they rarely school well. They do not feel threatened in species tanks, so only hang together is there is an artificial "threat" landscaped/aquascaped into their tank.
Anano's technique for fish in this subgroup is instructive. He puts a large school into a very large tank and has tallish plants only at the extreme ends if at all. A number of his early "Plains" or "Mountaintop: designs fit this scheme. These designs offer gently rolling contours of low to very low green over much of the length of the tank, offering no real cover for a large school of relatively small fish. If there are tall plants at the end(s), one or both, the fish will school while swimming the length of the tank and may break up a bit in the taller "forest egde" of the end(s) of the tank, but then will re-form for the return sprint across the "open to the sky" area which is most of the tank's length. The mountaintop tanks lack the end tall plant, but may lose some of the school among the standing stones of the very top of the mountain, off-center in the tank, if they provide too much shelter, but they will still reform for tank-length swims if there is good current running one direction along the front glass and the opposite along the rear glass. But this only works well in tank four to six feet long. In shorter tank, there is not enough space for the fish to form a coherent school. Neons actually may school a bit better just from being a bit smaller, but I admit that I prefer the more colorful cardinals. If you have relatively soft acid water, the neon school should last an average of ~10 years; the cardinal a couple of years less. In alkaline moderately hard water they may last a year or more less in age. Either tank is worth setting for a really striking tank showing what schooling really is, but it will need regular upkeep on the "carpet" plants and glass cleaning, but is actually easier than most multiple species heavily planted tanks.
Rasboras are much tighter-schooling fish. Obviously the harlequins are ideal for shoter (in length/width) tanks. They even smaller Rasboras may well be better, but tend to be much more demanding on water profile and quality. The will school well down to 30" tanks, but obviously 36" or larger is more showy. The really tiny Rasboras will school well down to 24" tanks, but are IME very demanding fish to keep.
So, "threatening" with open spaces can promote schooling, but the threat needs to be toothless and fit their inherited tendency to school tighter is open spaces or when predators are around. Obviously co-housing with a predator is a no-no. Any sufficiently large fish could be tempted to eat Neons, and would distort the apparent scale of the design.
Barbs are pretty good schoolers. Tiger Barbs among the best. They will school in smallish tanks, but the overlap of fish in both directions gets a bit chaotic. For some reason I burn out on Tiger Barb schools after about 3 years. I think that they may just be too exhausting to watch. I once had the Executive VP of a firm I was working for over - she said that I obviously did not sit and watch that 55 gallon Tiger tank for relaxation. I told her that in practice I could fall asleep in front of that tank faster than any other, as it was just fast repetition with few significant changes, so was in that hypnotic and stupefying rather than fascinating. But then, she already knew that I was just a bit crazy.
Where's the fish? - Neptune