Costume rules are at
http://disneyland.disney.go.com/faq/parks/#parks-q11
Adults are only allowed to dress up in costume for the Halloween party, but MAYBE all day party days (I forget what they did last year). You can certainly be in costume from the time people with the tickets are allowed in without separate admission (3 PM Tuesdays/4 PM Fridays).
The rules above are that your eyes must be visible, but you can wear masks. You CAN dress as Disney characters for the party! But people dressed as Disney characters are officially not supposed to pose for photos with guests (realistically if you are not mistaken for an official Disney character you are probably fine). I don't remember if the above link says it but the Halloween page says basically, not too gory/adult.