Yes, according to those who are going to be working IN the show (meaning Performers, not Cast Members assigned to the theatre), rehearsals start in September. For most professional theatre, performers are given about 2 months (sometimes less) to rehearse until the show is put on. I don't think the Soft Openings are going to be weeks in advance like they were for Everest. I think a soft opening to preview the show will be maybe a week in advance, perhaps even a couple days. If anything, they would want to get the show open to all guests in the park as quickly as possible.
As for the jokes and blah about the writer of Avenue Q doing it, I know what y'all were saying was all in good fun ^_^ But seriously, haha, the reason why they chose him to write the music was because he puts quirky jokes in his lyrics that people catch, and laugh at, and love. This was kinda what Finding Nemo was all about. The fact that almost every Pixar movie deals with an "out of the ordinary" subject (like Fish, Toys, or Monsters) and personifies them as humans was sorta what Avenue Q did.
Avenue Q was, haha "Sesame Street: Uncensored" if you catch my drift. The puppets (though obviously being puppets) were just like humans (including nudity, sexuality, etc etc.) and then there were those happy-go-lucky messages they added in there, and didn't sugar coat. It's like smiling at your friend, and in a cute, sweet, honest voice, saying "It's not that I think you're fat, I just think you're morbidly obese" and then breaking into song about it. It was these sort of things that were quirky and funny, and attracted Disney to the writer.
They thought "Gee, how great would it be if we could break into song in the middle of a moment in Finding Nemo? Wait a second, didn't Dory do that? How perfect!" And they chose the best writer for it.
Now don't get me wrong, I love Finding Nemo, I think the writer is gonna do amazingly, but I don't like Avenue Q. There are many reasons why, and as a theatre person, I can say, honestly "Avenue Q was great" but it's only directed at certain types of audiences, and while I can say it did well, their company cheated certain OTHER shows out of certain awards, and yet they're still not doing as well as those OTHER shows, who have been running just as long, and got to where they are now WITHOUT the marketing strategy that Avenue Q took.
Anyway, haha, I can't wait for the show to start! I'll be back in December, and LOTS of new things will have started.