Small schoolers: cardinal tetras
Medium: Bosemani rainbows (really rainbows in general)
Large: red hook silver dollars
Now for a huge tank/pond, I'd love to see a school of flagtails, but for them I hear it's either 1 or 10+
I forgot about congos. There are some other very nice African tetras as well. I really wish they were a little cheaper though-and the same goes for roseline sharks/barbs. They're like 3-5X as expensive as congos around here depending on size
Clown Loaches, Polkadot Loaches, Yoyo Loaches, Zebra Loaches. A mixed squad of these or all one type will get great shoaling behavior. You would need at least 8 individuals to get good shoaling. Clown Loaches get the biggest out of the ones I listed, but I like the Polkadots (Botia Kobutai) the best.
Small schoolers: cardinal tetras
Medium: Bosemani rainbows (really rainbows in general)
Large: red hook silver dollars
Now for a huge tank/pond, I'd love to see a school of flagtails, but for them I hear it's either 1 or 10+
I have two tinfoil barbs in a 270g tank and they get along great. One is about 12" and the other is about 5". The baby just follows the larger one around but eats like a pig.
I am planning on picking up 4 more silver dollars to add to my 5. I like the way they school and they look pretty cool as well. They get my vote.