I agree don't worry about the fin. My daughter who is 9 is on a 62, she sounds to be at about the same level. She was running the mini around 25 mph and now is moving to working in the full course. The ski size was recommended to me by a pro skier who has kids around this age. He told me not to go over a 62.
I disagree about skipping the mini course. It gives them early success, teaches them to pull across the wakes and they don't get frustrated