Can different species tetras form a proper school?

Not usually, I'd only imagine cardinals and neons schooling together. It's interesting that your x-ray is schooling though. Even three of a species is on the low side, however. They tend to prefer 5+. I'd try getting the 3 x-rays. It's too bad that that is only a 10 gallon. If it was bigger, I'd recommend getting 5 of each, and a few more frogs.