I really think you are setting them up for someone to get hurt if you keep them all together or even separate in one duck one drake pairs. Yes there are anomalies where it works but it's generally not the rule. Especially when you already have one drake you say is aggressive and the other drake is quite larger than the females you have. And I know you said the khaki drake gets along with other drakes but the pekin doesn't, but I don't think you're considering that the khaki is still a baby and this could easily not be the case when he's mature. Getting him another drake friend might not work when he's older any better than introducing the pekin to other drakes has worked for you, and every day he will hear the females he grew up with and they will all be wanting to be together because they are a flock.
And I know you don't want to re-home anyone but you have to consider that if you don't find a good viable solution you may be looking at losing one of your ducks to death instead.