This is it, though. Pikachu wouldn't get a mega for the exact reason that it's the big mascot. They wouldn't want to change its image to plug something like mega evolution, because then we're stuck with it. They can give it new costumes and interchangeable looks for something like a contest, because they can always do away with contests again in the next games. And we'll still have plain-old, adorable little Pikachu.
As far as seeing the different contest forms in Poke-amie, it probably means that Amie is getting fitted to display your Pokemon's contest stats on top of its usual functions (Amie could become a function on the Pokenav, even), and if we can add accessories to other Pokemon, you would go to Amie to select a new default wardrobe, or something similar.