Be Our Guest only has Beast and only at dinner. Mary Poppins is at 1900 Park Fare at the Grand Floridian with Alice, Mad Hatter, Pooh and Tigger, but only for breakfast. Dinner is Cinderella, Prince Charming, Lady Tremaine and usually the step sisters but sometimes it is a mouse or fairy god mother instead.
The princess meal in MK is in Cinderella’s castle (restaurant is Cinderella’s Royal Table) and has about 5 princesses at all meals (as far as I know). There is also a princess meal in Epcot in Akerhaus (Norway) which has about 5 princesses at all meals.
A pre opening breakfast at Crystal Palace may not get you to Merida in time to avoid a long wait. She is very interactive so the line doesn’t move quickly. I’d go straight there at 9, she starts meeting at 9.15 I believe, and do breakfast around 10-10.30. She doesn’t have FP but you can FP Ariel and the princesses in Fairytale Hall - I don’t recall who they are, they meet in 2s. Anna and Elsa are over in Norway and don’t take FPs but have a short wait. Other princesses meet in the countries at Epcot but outside.