Like Rosetta said Tiana is a very specialized princess. When I was an attendant I knew someone who had been fitted for her but disapproved because she was too thin. This sounds really odd but performers who are friends with her have to have a certain skin color shade. Also from what I understand her friends tend to have larger foreheads as to accommodate her hair. I can't really verify the last two statements and have potential to be totally rumors. However I did hear them from within the entertainment department. So basically take them with a grain of salt.
When I started my program back in January I know that were still in search of more friends for Tiana because at the time there were only 3 or 4. A year later I'm not sure what the situation is but I would definitely recommend auditioning. You never know what can happen.
The body size thing relates to all characters, not just Tiana. I've read several accounts where people trying out for different face characters were turned away because they were too thin. If you're too thin you can't fit into the costume. I also read that most of these performers are either a size 4, 6, or 8. If you're smaller than a 2 you're out of luck.
Is Tiana's wig shaped differently from the others or don't all face character performers need relatively large foreheads for their wigs?
What you said makes complete sense, but based on all of the pictures and videos of meet & greet Tiana that I've seen I'm going to say that the "must be a certain skin tone" requirement is either not true or it wasn't always true because I have seen several pictures and videos where Tiana is too pale. The lightest Tiana that I've seen mass photos of is this one:
She's about the same complexion as Tyra. She's also close to Naveen's complexion and anyone who has seen any pics of animated Tia and Naveen knows she is no where near Naveen's shade of brown. At least one friend of Aladdin also gets to be friends with Naveen (or whichever one he was friends with first). Her nose is also too pointy, instead of it being a button (flat and round). I am surprised that she was in so many promo pics for the character. NO, I am not trying to be mean or dissect her, I just like the characters to look as close to their film appearances as possible.
If this "rule" is in effect now I am all for it since last year it seemed like so many Tianas (though there's only one) had noticeably different skin tones and facial features that it made me wonder if they even tried to cast her accurately. You would NEVER see a tan Snow White because they cast her appropriately. On the other hand, I have surprisingly seen pictures of a couple of Paris Cinderellas that are noticeably tan. Just a couple more shades and they would match the color of their blonde wigs.
It makes the most sense for Tiana to look as she does in these pics:
The 2nd pic isn't that big or clear but she's in several youtube vids where her Naveen is acting crazy like Naveen does and is supposed to.
To the OP: You never know until you try so it's always good to audition. Just note that in WDW if you are a face performer you must also be fur, though you could end up spending more time in face than in fur.