With that many females a drake shouldn't be too much of a problem. He may even be appreciated by your girls as ducks have pretty healthy appetites. I had 5 girls (down to 4 now) and there haven't been any issues.
Hah! Back when mine were smallish i tried this because one of my rouens developed a reslly dark almost green head and i thought she might be a boy. The problem is that she was so sweet and docile even of i removed out of sight for several minutes she wouldn't make a sound. Just sit in my arms...