I don't think they guarantee any specific princess's will be on any particular sailing.
When we were on the Wonder back in October, my DD met/did pics/got autographs from at least:
Snow White
"plain" Belle
"fancy" Belle
Cinderella and Prince Charming
Princess Aurora
Mulan
Wendy Darling
There were probably more, I just can't remember more.
Every day in the atrium/lobby, there is a list of which characters will be posing for pictures/doing autographs and when and where they will be.
They also did a grand - "Princess Greeting" where the girls literally went from 1 princess to the next to the next. This was a hugely popular event, and I would recommend getting in line at least 1/2 hour early. I believe they "cut" the line as soon as this particular event started.
My definition of "cut the line" is everyone who is in line will have an opportunity to get the autograph, and pics, however, at a certain point for every character greeting, a cast member will not allow any new people to stand in line.
I personally was so impressed with the princesses, as they talked to each girl, and really engaged the girls in conversation!
Have a great trip!