I can't guarantee any of this is still true, but the last time I was there (end of 2011)
Mickey and Minnie both met out by the train station several times throughout the day, as well as in their individual houses in toon town.
Princesses were available at their own dedicated meeting spot just outside toon town. They rotated throughout the day, 3 at a time. Here, I have encountered Cinderella, Ariel, Belle, Tiana, Pochahontas, Mulan, Snow, and Aurora.
I also ran into princesses just strolling on their own near the castle/snow white's wishing well. (Aurora, Ariel, and Snow)