
I am new to having both Ducks and Chickens together. We originally started out with four Isa Brown hen chicks just for the purpose of a few fresh eggs on hand and then two small breed Mallard hen ducklings. Then four weeks into it we decided to add ten unsexed Rouen/Pekin mix ducklings which produced four Drakes and six Hens.
So far we have been very lucky and everyone has been getting along very well for the most part. I am in the process of giving the four chickens their own Coop and run and separating the four Drakes from the Hens because of incorrect ratio, just to help avoid any issues.
I agree with HuffleClaw that you don't want your Drake to try mating with your hens because of the possibly of hurting them. Since limited space is an issue for a second coop and run, you may want to re-home your Drake and just keep all girls to help avoid any mating problems.
The other issue I see with adding new birds ( chicks or adults ) to your existing flock, this too can cause problems. You never want to introduce new baby chicks into an adult flock unless they are their own babies and momma should protect them from others.
To help avoid any issues by adding new adult birds into your flock. Quarantine for awhile is a good practice to make sure no one is sick. Then it's a good idea to find a way to keep them separated within your coop and run for away so they can all get used to seeing each other first, over just throwing them right into your existing flock.
Birds are no different then any other animals that live within a small group, they have a pecking order also. So just throwing new birds into your flock without proper introduction can also cause unwanted issues.
Good luck and please keep us posted.