You won't need anything that will be too extreme, but I would go with like a emporer bio wheel 400 and a aquaclear 300/70. The aquaclear 300 also goes by the name of aquaclear 70. They are both pretty cheap and great filters. GL
Well... the silver dollars surely won't be harassed or in danger. The live food, er, female bettas will be in SERIOUS danger. My silver dollars LOVE female bettas... and male bettas too.