actually, aquariumfishguy, I have known ppl with bigger tanks and multiple bettas in them that were fine for a period of time, then something happens and one fish either kills or injures another. Usually females are ok together, but I personally myself wouldnt put a male with any other betta. I have some males that harass any other fish, betta or not. I got one now that just killed an oto. I dunno, mabye its me, maybe I got some bizarre violent bloodline or something.
Just my experience with the fish.