I had similar problems with mine, I had read that more is best with Serpes so I put 10 into my once peaceful community tank and then watched them nip at every single fish in there. Its a 60gal long so they were not crowded at the time.
I moved them out into their own tank and let them fight amongst themselves but they seem to have sorted a pecking order out now and are behaving in a species tank. But the tank isnt really big enough for the 10 of them and needs a lot of careful watching and maintanence.
End result though is that as we are having trouble with the
betta tank maintaining an even temp we are going to return them to the LFS and move the Beta into their tank he will enjoy the extra 10 litres I am sure.