Funny that I just read this post cause ironically enough I just had a princess yesterday that came in with cornrows in her hair (granted it was only maybe a section of the front of her hair but I digress) and beads as well. It wasn't terribly hard to do her hair style (she got the fairytale princess), I just worked the cornrows into the style (left them in, beads and all and pulled the rest of her hair up into the pony tale and formed the bun from there). Because of the cornrows I was somewhat limited as to how high I could get the pony tail for the bun but it still looked pretty when it was finished. Not to worry, I'm sure the FGiTs can work with your daughter's hair and if she can't get one of the hair styles they can suggest a couple that will still look cute with the cornrows in (i.e. the Pop Princess with the multi colored hair or the Disney Diva with the sort of "princess poof" and the curly extensions at the back [i.e. a Hair piece with hair to make the poof and extensions attached]. As far as substitutiing the hair for the nails, your package comes with hair makeup and nails but the kicker is that if you just get hair and makeup done and no hair its still going to be the full price of that package (i.e. 59.95 and so on and so forth). I would go in and see if the FGiTs can work in the corn rows (which they usually can) and then go from there. Doesn't hurt to try!
Aaand to answer your last question about the tiara, the crown is $10.95 by itself. They come in Silver, Blue, and Pink.
Hope this information helped!!! Lemme know if you have anymore questions!!!
Meredith