Desert is increased only by the "hot" setting. The wet-dry doesn't factor into it at all.
Whereas, I believe that Marshes are increased only by the "wet" setting.
So for a map with the most Deserts and Marshes, you should go Hot-Wet-5million age. Also I have noticed that larger maps have more of the same terrain-type in the same area, so if you're not otherwise opposed to it, a Large or Huge map will give you a Capital with the most Polder squares in it.
And clearly, the Flood Plains start > Marsh Start. Not only is it one more Food, but Desert starts are just OP thx to Desert Folklore. For that, there's a mod called "Really Advanced Setup" that allows you to give your Civ a start bias (among other things). Although be warned, mods will disable achievements and non-random starts are a bit taboo among certain players.