Adults may dress up at WDW during Halloween parties at the MK, but not at other times and in other parks. You can't cover your face and it needs to be....ummmmm.......a family rated costume. We heard them tell a young woman in a very revealing Tinker Bell costume that she had to cover up or leave at a party last year.
One year there were three ladies dressed up as the three fairies and they had a line going and were doing photos and autographes. A CM stopped them, telling them that they did not want the guests to be confused about Disney vs guests.
I suspect the same rules apply at DLRP, as the wording about young and young at heart is the same as at WDW.
I dress up every year in my homemade Fairy Godmother costume and fool guests and CMs alike. It's a lot of fun. I am always asked for photos and autographs and I will do some photos as long as it doesn't cause a distraction for other guests. I have had excited mom's chase me down with an autograph book more than once, and sometimes have to work to convince them I'm not the "real" thing. But it's just too hard to turn down a hug and photo with the little ones, they are so darn cute when they see me and get excited.