minimum schooling size?

I currently have 3 white cloud minnows (want to get a few more, but I would be overstocked) for my 10 gallon, and they often will swim together, but usually they just chase each other around.
 
I have 4 Purple Emperor Tetras in my 10 gal. right now. They school sometimes but not all of the time. They seem to have a little trouble establishing who is the boss. I'm adding one more and moving them to a 20 gal. long tank next Saturday. I'm sure they'll be thrilled.

I wanted to get a small school of Cories, maybe four, for my 10 gal. but I have Florite substrate. I heard that they need sand substrate. Is that true, or could I get them for my tank? I was going to get White Cloud Minnows instead. Are they fast and active like the Tetras?
 
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