A secret submission, like Melon says, is a great way to keep the fighters in the dark. It prevents the building of characters "immune" to everything that others can do. You have your parameters for what constitutes a "character" acceptable to this fight, evaluate each character along those lines, if there is a question about something send it back with your concerns. You are the ultimate judge of this. As I told you I can help evaluate the characters as a second set of eyes, but if you feel that a character does not fit your parameters you can deny it with the caveat that the player tweak the character.