I don't think it's fixed. Belle is almost always the main princess (though I've heard another princess is sometimes used on rare occurrences) and they do photos with her as you come in. There are usually three other princesses that walk around. We've had (on different occasions) Ariel, Snow While, Aurora, Jasmine and Mulan.
We have done this for breakfast and lunch. We saw Snow White, Aurora, Jasmine, Belle, Ariel, and I think one time we saw Cinderella. Great experience, lots of one on one time with Princesses and the food is really good.
We had breakfast at Akershus this past May. Belle was out front for the photo then we met Cinderella, Snow White, Aurora and Ariel inside. The food was just OK, but my DD's (3 and 6) loved it. Have fun!
We have had Belle each time we went (5) as the main princess for the picture. DD's LOVE it We usually see Snow White, Jasmine, Ariel, Cinderella, Aurora and once Mulan.