I just remembered something. First I had one neon blue (has pretty red stripes over bright blue) and then added a powder blue and a neon green (again red stripes over green) the green was so aggressive I took him back the first day. I still have the two types of blue. IMO
in a 30g two would be fine, but once you get the right two. Don't be afraid to ask your LFS
if you can exchange them if they don't mesh well. Also, try to add them at the same time, someone here mentioned that it would cut down aggression because one wouldn't have established its territory before the other.