I would be cautious of serpaes just because they are such well known nippers. I have 13 and they have only nipped eachother, but I have also never had anything with really long fins in the the tank with them.
I'm sure some of these have been said... but these would all work:
Von Rio (My favorite), BloodFin, Bleeding Heart, Buenos Aires, Cardinal, Glass Tetra, Glowlight, Pristella, Lemon, White Skirt, Black Skirt, Black Phantom, Red Phantom, Diamond, Emperor, Rummynose
Hatchetfish?