Hi Adams, about at 0:55, i'm not pianist but i have that feeling that a note goes a tiny bit slighlty off there.
About the mix, you may have center a bit the piano but the difference is not that much noticable because it's stereo effect is quit wide. I would have decrease just a bit it's wideness and thus giving more space for the upcoming instruments on the sides.
I like how you placed the flutes they sound great there, but the brass are too much panned to the left in my opnion, they are too much isolated from the rest, i would have panned them just as you did for the flutes.
For the strings i would have made them even more wide, just by using a good chorus ( you can found some good patches for in the edit insert in the ssl mixer )and eventually use mid/side processing and as i said to fullfil that stage from wide left of your stereo imaging to far right, just like the strings were supporting all the others instruments.
Now if you notice at 2:09 when the flutes and brass kick in, your strings almost disappear. That why i was talking about giving them some space and presence. Also equing your tracks will help to separate each instruments and prevent some frequencies to fight each other for the same place in the frequency spectrum.
Everything i'm talking about arrangement in the mix here is just my personal opinion and how i would have mix it, opinions aren't rules.
If you didn't watch the Learn Reason "how to mix" video's hosted by Matt on Youtube i strongly advise you to do it, it will help you to have a different insight of the mixing work.