My male betta paid my cories no mind when they were in the tank with him. I moved the cories out to give them more room, but he never bothered them, and the cories never bothered the betta. If you are really concerned, spring for a Veiltail or a Halfmoon (Doubletails are great too) as they tend to be more docile than crowntails and kings. One of the Betta people also said plakats are calmer bettas as well