I had the string quartet. To my knowledge Disney hires these people from the outside from like a preferred list. They get emailed your music and they practice on their own. To my knowledge they did not practice together prior to my wedding. They did not come across like I would have liked in my video. I too was excited about live music, but if I did it again, I had to pay for an audio tech, I would have just used canned music. I think it also depends on the space on how it would sound. If you are going to put out the money for this. I would definately ask to speak to entertainment to make sure that you were all on the same page and what their recommendations are. I never did speak to entertainment, and I really should have insisted to do so.
I don't know if it would be worth it to try and hire the pieces from outside of Disney or if they would allow that.
I will say that the violinist that plays at the intimate weddings is AMAZING!! I highly recommend him, but I can not remember his name right now.