I have 8 cardinals and 8 rosy tetras in a 180l tank. At first they schoolled but as they got used to the tank they just did their own thing.
I added dwarf gouramis and this made no difference. There's always been guppies there, they are quick and mess around but the tetras are fine with them.
Recently I added some young diamond angelfish. This has made a difference. All the tetras school in their own groups and swim about the tank. The angels are natural predators of some tetras, but I've heard that if they grow up with them they should be okay and not be expected to snack on them. Here's hoping anyway.