12th Fleet prefers Star Trek Canon races but does accept semi-canon and non-canon races ((Semi and Non-Canon races must have a logical explanation to fit in the Trek Universe)). Please check
http://en.memory-alpha.org/wiki/Species_and_cultures for some ideas. Remember, RP is about creating a character, for this reason weaknesses are a necessity. 12th Fleet has the right to refuse characters with superpowers, limited weaknesses, mental powers or with character concepts that do not fit the ideals of Star Trek.