Well, it depends on what they are looking for at the time of the audition and what roles they need to fill in the parks. It's not necessarily required to be trained in ballet, but I do have to say the more versatile you are as a dancer the better.
Nonetheless, you should definitely go to the audition no matter what! Think about it? What have you got to lose?! The open calls are free and if they are interested they will call you back. And keep going back and auditioning each month because they are always casting different things. So, sometimes its about being in the right place at the right time.
Also, if you are really interested in Disney I would recommend looking up the Disney College Program, where they can go to college and intern as a performer. It's a pretty cool program if that's what your into. :)
I've had multiple dance teachers perform at Disney Parks, and have attended many Disney Performing Arts Workshops. :)