We used one of Rick's suggested tour guides for the Uffizi and Accademia Gallery tour and she was amazing! Her name is Elena and she's with http://florencewithflair.com/. Not only did she help my mother get a wheelchair at each of the museums, she also helped with crowd control and was the "rude" person to tell people to get in the correct lines and let us wheel my mother to the front of a work of art. She is highly knowledgable about all the pieces she showed us as well as other works that we had questions about. She also called us taxis for the trip in between for our group of 7, showed us the best gelato places near the museums and called taxis for us to leave after our day trip was done.
If you are traveling with someone that can't walk much and needs some special assistance, I would also recommend doing the eco green golf tour. You sit in a cheesy golf cart and it's a bumpy ride, but you get to see so much and they also have English recorded info about all the stops. It was a rainy day trip for us so not having my mom walk around on the slippery cobblestones and trying to bust through crowds was a blessing! I would recommend the 2 hour tour that takes you up to Piazza Michelanglo for an awesome view of the city!