Merida isn't on any of the regular schedules so you'll have to check the site for her times or just walk by her area. She's AMAZING so if the girls are a fan of her, definitely go in!
A met a ton of princesses, but we only really planned to meet Rapunzel and do the CRT dinner and that was it. We had to replan everything on the fly when we realized the only thing she wanted to do was meet princesses. So I guess the moral of the story is BE FLEXIBLE!
Castle Couture is behind the castle where BBB is. At the end of BBB they sprinkle glitter into the girls hair and its "pixie dust" but you can walk in and have them put glitter in her hair for free, without an appointment. Of course, you'll have no help getting the glitter back out of their hair, LOL.
Also in DS there is supposedly a photopass studio where the kids can dress up in different things and go in front of different back drops and you get all the pictures included with your photopass. We didn't get a chance to do that on our last trip though because it was so crowded.