I've read a couple of times before that generally, the larger the number of individuals in the group, the less likely they are to be nippy. Don't know what level of truth there is in it though.
The two species you mentioned (neon + glowlight tetra) do better in 15+ US gallon tanks, so yours is a little small IMHO. What about microrasbora, ember tetras, pygmy cories or endlers instead?