I have 2 zebra danios (was 3, but one mysteriously disappeared from my son's tank, so I relocated the remaining 2 to my tank!!) I also had a school of 6 zebras when I first set up my tank 4 years ago (they were my cycle fish... I didn't know about fishless cycling, and forunately they all made it thru the cycle).
While it may look like the danios are chasing the others away, it's really all just in play... it's not aggression. My school of 6 always had one (don't know if it was the same one, they were pretty much identical!) that started the game of "case and tag" thru the tank.
As far as what can school with zebras, I think anything goes. In the sense that anything can go in the tank w/ them, but I don't think any fish will actually "school" with them. They tend to stay closer to the top of the tank (exception of when they are darting aroudn playing chase). I had a betta in the tank at one time, but had to remove him, as he didn't seem to appreciate the zebras butting in on his little area of the tank.
~Tara