The thing to remember about Alice is that she LOVES fashion and cute, girly things - remember how she likes to make Bella her own life-sized Barbie doll?
I think what would make Alice costumes stand out from Rosalie costumes (as far as clothes go) would be that Rosalie would go for more elegant clothes that would accentuate her beauty, whearas Alice would go for something more fresh and funky looking. Think Abercromie vs. Hot Topic - Alice would be HT.
Is there any way you can do your hair to look like Alice's? If you already have shorter, dark hair, you could easily get some hairspray or pomade and make it sort of spiky, like Alice's. The hair is really going to make all the difference with a costume like this, where there is no real "uniformed" look. It's not like the Harry Potter parties, where everyone's in a schoolgirl skirt and a gold-and-red-striped scarf. The Cullens really don't have a specific brand or look, it's more about how you interpret them.
I hope I could help some - have an AWESOME time at the party, I can't wait to get a hold of my copy, either!