Good stuff Grace! I also agree with Gilbert on implementing legumes in your polyculture, they are nitrogen fixers. You're going to have plenty of organic matter if you chop and drop. Sunflowers will help your corn, and pumpkins are cucurbits so they will also work well in a three sisters guild. Your potatoes will benefit from horseradish, peas, & garlic.
This got me thinking: Should one change cover crops from season to season when planning a crop rotation? So let's say you plant your three sisters garden, and after harvest you chop n' drop, then plant clover as your cover. In the next year you switch from corn, beans, & squash to brassicas. Are you going to want to change out the clover for a cover crop after the 2nd year's harvest?