I've always found it best to look at the bird from the side, in profile, to try to guess the gender. I have this page on my website that describes the differences.
http://www.muscovyduckcentral.com/gender.html The caruncling on that black pied bird makes me wonder... Usually ducklets will be nearly feathered completely by the time their caruncling starts to appear. I can't quite make out whether the black pied has all of her primaries in? That would be a consideration for me as ducklets get their primaries in long before drakelets do. There is an obvious difference in foot size which could be due to the difference in age as you mention. A photo in profile will help...