as long as you put the rules in the con book, like if there is special attire or what not
last year i wasn't allowed in the 99cent costume contest becuase i was wearing a costume and aparently covering it up with a new costume wasn't allowed, and somthing like that would have been good to know in the description in the booklet (not that it would have made a differnce my street clothes were locked up in a friends room)
but unless your not going to allow peeps in cosplay, i think the only thing to recomend is for people not to wear baggy cloths or cosplays with little bits that could get stuck or ruined by duck tape