I vote that you have Hermione as one of the seven. She's powerful, and makes Einstein look like he never passes 5th grade. I agree with the Fleur and Tonks, I can kinda see the Daphne, but Luna has always been a stretch for me. I can see Susan if you have her be quiet but meek like Maxtaf said. But personally You really should have Hermione as one of the seven. But your story. As for a use and throw away girl... Pansy. Perfect revenge on Draco.