I very much agree with increasing your females if you get a drake. The 3 girls I adopted were over mated and even 5.5 years later one has health problems requiring surgery under anesthesia yearly to save her life. She is a 7lb Pekin and mated by little mallards. My ducks had been abandoned on a pond, and there were many mallards - but I have read plenty of tragic stories on here where a single drake killed their partner.
It may be fine, but the new drake could severely hurt or kill your girls and no one can predict what will happen. Personally, I wouldn't risk it.