Just because you're dividing it doesn't mean you have to so evenly. You could maybe do a divider 1/3 of the way across, with a little over six gallons for you're aggressive guy and a little over 13 for your more passive guy and a nice school. From what i've read, the more comfortable a schooling fish is, the less likely it will be to harass your betta, so more is often better to negate that risk.
Peter - Blue CT - 08/16/2011
Pero - Dark Salt & Pepper Miniature Schnauzer