I would advise to keep two for the fact that if keeped as an individual they tend to chase/harrase other fish. Get them when they are small and once they get to big for a 30G tank move them to a larger tank or take back to the LFS.
I'll second Trenac suggestion. I've had two in my 20G for the last 5 months. I had one alone for a month prior to that and he was chasing the neons around a bit. Now the two school together and leave the other inhabitants alone.
Size wise they are not a fast grower, my first one is still just over 2".( was about 1-1/2" when I first got him.