Hi Janet!
Disney's Grand Floridian Resort and Spa is definitely a resort in a class of it's own. You will be transported into a Victorian beach resort, right down to the beautiful music playing in the lobby and the costumes of the Cast Members. It's truly an elegant and gorgeous resort.
That being said, you can most definitely wear shorts and sandals at the GF! I have always felt comfortable in my theme park or swim gear at the Grand. You will see guests in every style of dress, with casual resort wear being the most common. The only dress requirement would be at
Victoria & Albert's, a fine-dining establishment at the GF.
As for Mickey's Not-So-Scary, you will see adults and children dressed in fantastic costumes ranging from the elaborate to the most basic ensembles. There are also plenty of guests in normal park gear, Halloween t-shirts, and everything in between. No matter what you chose to wear to the party, you'll fit right in!
Please let me know if you have any other questions about your Fall vacation. I'd love to help you make it as magical as possible.
Pixie Dust,
Ashley