I was planning to plant a Nules Clementine about 20' away from a Yosemite Gold mandarin. This won't add seediness to either fruit, will it? My understanding was that only seeded citrus had this effect on mandarins.

I don't think that the two trees you mention would produce much of any seeds no matter how close together they were planted. Yosemite Gold mandarin is genetically a triploid mandarin, and will not produce seeds even in mixed plantings. Nules mandarin is a diploid mandarin. It is "possible" for a triploid seedless mandarin such as Yosemite Gold when crossed with a diploid mandarin such as Nules to produce seeds, but it would be in the range of none to a very limited number per fruit (maybe 1). However, to another point. If you were planting two varieties of citrus prone to seediness, 20 feet apart would not be far enough away to avoid cross pollination, thus seed. - Millet (1,096-)

Thanks!