Drakes if kept alone with no ducks, will try to mate with chickens, but if your bantam can fly at all, like i know some bantams can ,it shouldnt be a problems.
if you keep 2 to 3 ducks for the one drake hell have no time for the bantam
in my 12 years of raising ducks, which isnt considered a long amount of time, the only time ive seen males be agressive is when they do not have a heram, which is a group of one or two drakes with 3 to 6 ducks in a group , those males with out this did have to chase and ultimatly rape the females belonging to a heram. But if you keep the right number of male to female ratio. you should be fine.