Scott, it's easier to run the wallboard to the floor and apply the Kerdi, then place the mud and Kerdi the floor.
The minimum size joint depends on a couple of issues: the maximum variation in tile size, warpage of the tiles and flatness of the walls. Use 3 times the maximum variation as a start, then add 1/16" or so for warped tiles, and again for non-flat surfaces. Keep in mind that DIYers find slightly wider than minimum lines easier since there's more grout to hide your mistakes. Grout is your friend.