Adults in any costume are not permitted into the parks during normal hours any day of the year.
During the MNSSHP event, adults may dress up, provided the costume is not gorey or obscene, and they don't look too much like any of our princesses.
If you show up to Epcot at noon on halloween day dressed as Batman and Supergirl, you may run into some difficulty.
My advice is to dress normally if you plan to spend most of your day in the parks. When it comes time for the Halloween party, head back to the hotel and change (you will need a break anyhow if you're going to party it up at MK).