The value of potato peelings, banana skins, etc., is not as a direct food but because they encourage invertebrates that the ducks then eat.
No need to mash the tatties - put them in whole. They will then decompose at different rates and provide long-term benefit.
As far as grain is concerned, barley seems to be preferred but no need to buy good quality grain. The sweepings from under the driers will do fine.