There have been several instances where people had swans and kept koi in the same pond. However, due to the size of your pond, we would suggest that you stay with the smaller species, the Australian Black Swans.

Since you have ample plants and filtration system, the nitrogen levels should not be that big of an issue. Most fish, especially of the carp family or bottom feeders eat fecal matter found in bodies of water. The Regal Swan