Chris is my original zebra danio from the day of starting the cycle in my ten gallon tank. That was in May of 2003. He was part of a pair, but his friend left us in summer of 2005.
Last fall I changed the ten over to a twenty long. Chris is still perfectly happy, and has had a new friend since last summer.
Conclusions:
1) Zebra danios are indeed a schooling fish, but I have noticed they live perfectly well both alone or in a pair. So all of those who make these rigid claims that it's wrong to keep them out of a school are just being purists and not going on anything but some sort of conventional ideal.
2) They seem no less happy in ten gallons then they do in twenty. That probably goes for any other size tank. Yes they are active, but they really don't require a lot of space to be active. They are very tiny fish.
3) They truly are hardy as hell.