This is a good 'test'. Keep the aggressive cockerel separated for at least few days, if not permanently.
Having multiple males can definitely be problematic... the competition factor makes already hormone driven young males behave at their worst. If you are new to chickens, then having zero males might be the best route for you.
Adding more birds is another learning curve that I would not suggest you leap into just yet. Yes, lone crested birds can have problems being picked on, but it doesn't always happen.