A lot of the fish people are quoting on here as 1.5 inches or less aren't, such as Zebra Danios, or Neons.
If I were going to do a mixed tank for a 10 gallon, I would do:
3 Endlers - One Male + Two Female
6 Pygmy Rasbora (seen them at LFS...wonderful creatures)
6 Ember Tetras (also saw at LFS, but didn't have enough for good shoal)
6 Pygmy Corys (top out at an inch)
That pretty much fills up the water column (you have top-dwellers, mid-level, and bottom) and comes out to close to 20 inches of fish (and 21 fish!) in 10 gallons of space...two inches per gallon is doable if you're experienced and you plant the tank.
Personally, I like the idea so much I might do it myself. It's about the smallest complete community I could imagine.