I wouldnt worry much about cross-pollinization of different sweetcorn varieties, after all, they are all sweet. You do want to keep it away from fieldcorn however. If you plant 65,72, and 85 day varieties all at the same time they will not cross-pollinate because they tassle at different times. When you plant a faster variety later than a slower variety you can get some mixing of the two, but again, it wont hurt nothing. I stick with slower maturing varieties for later plantings since they tend to be better tasting anyhow. I also mix up the ground my corn is planted on, rotating (2) years clover, (1) year corn. That minimizes fertilizer needs for the corn.