For MNSSHP specifically? Or just the whole day?
For the party, there are no real rules, other than no weapons (obviously) and nothing that will impair your safety on the rides, or the safety of others.
During the day, I don't think they'd typically allow an adult dressed like a character you'd find in the parks (no impostors walking around). But other than just using common sense, I don't think there are any hard and fast "rules."
You may want see if your coworker has a specific idea in mind... and then we might be able to help in more detail.
Yep, I saw CM's have to crack down on the above. A guest was dressed up like Jack Sparrow and looked very authentic. He ran afoul when he started posing for pics with people. Staff stepped in and said something to him.