Yes- they can conspire with the Godmother in Training to surprise you with the dress choice. My DD, now 8, has done this very thing twice. I have just told the Godmother in Training when DD told her that is what she wanted to do, that DD could pick whatever dress she wanted, and off they went. They do usually ask you what dress and size when you book, but the child can change their mind and get a totally different dress once they arrive. (They ask supposedly so that they can try to be sure they have the right dress and size on hand for what the child wants- but there is no guarantee that will be the case) One time, DD just let me be surprised with her dress choice before the hair and make-up, but once she wanted to surprise me with everything which meant that I did not get to take pictures of the makeover because she made me keep my eyes closed. She loved conspiring with the Godmother in training to pick the "surprise" dress. They do usually ask you what package when you book, but you can also change packages- last year we had the castle package booked, but she saw the Minnie Mouse Halloween dress and immediately wanted the special Halloween package instead. I was thrilled because I got out much cheaper than I do with the castle package. As far as the dresses, do you plan to cart all of the dresses you already have to the BBB? She is probably going to want to leave there dressed up. If she looked at all the dresses there and decided she did not want one because she liked the ones she had at home, I think they would let you do the cheaper package. Unless you bought the dress at WDW/DL, or at one of the times where the Disney Store Website had the "parks" versions online, the dress at the BBB will be different. They are not the same as the Disney Store dresses. When DD was 3, she just HAD to have the Disney Parks version of the Cinderella dress at BBB, even though we had the store version already.(She was a total Cinderella fanatic at that age- still mostly is). I don't know about the FGIT helping with the dress. I think usually they don't. When DD wanted to surprise us, she mostly just wrangled herself into it on her own. I do recall one time where she was yelling for help, but was telling me I had to close my eyes to help, and I'm pretty sure the FGIT asked me if it was okay if she helped her, and I said it was fine. I'm not sure they are supposed to do that though. They do, and they usually get some great ones, so I would say that is a good plan. Just be aware that some photopass photographers are better than others. If they aren't taking enough pics, don't be afraid to ask them to take more. We've mostly had good ones who took a bunch of photos during the makeover, but we had one photopass photographer at BBB who was far more interested in chatting with the FGITs around the place and the parents than in actually taking pictures of the kids getting makeovers. He was barely taking any of the BBB kids in his area during the makeovers, and was pretty much just doing the reveals. I finally just went up to him and asked if he would mind going over and getting a few more pics of DD because we had pre-purchased the photopass disc and were really hoping on getting a bunch of BBB makeover photos. That was all it took to get him over there. If you have issues with not enough photopass pics being taking, just go up to the photographer and say your DH has your camera at PL and you were hoping to get a bunch of MM photos and ask if they will take more. We had one who was absolutely fantastic and took a ton of pictures during the makeover. DD was refusing to let me take pictures of her makeover with my camera that day for some reason, so he walked over to me, whispered "camera", and turned around and stuck his hand behind his back to take my camera, and then stared taking clandestine pictures of DD with my camera as well as his own. It was pretty hilarious, how he would wait until she looked away from him and then would pull my camera up and shoot, or would hold it off to the side where she couldn't see what he was doing and shoot. He was totally playing "superspy" and having fun with us. Hopefully you will get one like him!
It's such a WONDERFUL experience for ALL!!! 
Would love to book an appointment for my DD's 5th birthday. Is there a time of the day, or day of the week, that I may have a better chance at it? Thank you.
