Is this for the 30G?
If so, anything that won't fit in their mouths:
cory cats
various smaller loaches (zebras, chain, dojos, kuhli etc)
larger tetra (orange von rios, the skirt tetras etc) although mine don't mess with any of my neons, folks don't suggest housing them together *shrugs*
It also depends on their attitude.
I have mine in a 115 so my stocking is neons, Australian rainbows, Madagascar rainbows, dojo and zebra loaches, rasboras, albino bristlenose pleco