I personally do not see a need for a 6 month rule. A book is only fully nominated after getting a 2nd and a 3rd. If people are not interested in reading it, or find it to be out of genre, then it will not get nominated. If it gets nominated, then there is a interest, and denying the nomination says that we do not care that there is an interest.
Storm Front, like Lolita, was a "political game", for lack of a better phrase. People nominated and/or voted for it to make a statement. As I stated earlier, I'm pretty sure Storm Front would be ignored if it was suggested for the Romance category.