Yes, a drake and his harem can be housed with hens under the right circumstances. I've got a Mallard drake and 8 females who live with our 28 mature hens, and there have been zero problems thus far. I guess this also depends on the temperament of your boy, whether he's aggressive or docile towards other birds. Marshall, my own drake, is a naturally quiet, calm guy, so I'm kind of thinking that this is why everything is so peaceful in our flock.
I hate to say this, but two females won't be enough to appease your boy if there are also going to be hens around. The general ratio is 1 drake to 5 ducks, so if you would like to keep the two little groups together, you might want to consider getting a few more girlies to keep him happy. One last piece of advice: don't raise the drake and your chickens together, whatever you do! He will only see them as "his" when maturity hits, and then there will most likely be big problems.
You're correct in that although chickens and ducks may reside in the same area, they really don't intermingle for the most part. Mine get along beautifully but rarely spend time together.
Enjoy your babies!