Well, hope this wasn't too hard to figure out.
Phantom- Kaname (tall, dark, handsome; yeah that's our pureblood)
Swimsuit Model- Aido (only he would have enough self-respect to wear that)
Dragon- Rima (she has to be the only female character I don't dislike, plus she could totally pull this off)
Blue Meanie- Ichiru (my love for the Beatles has come through, thanks Ichiru)
Fairy- Yuki (sorry, kind of obvious)
Red Riding Hood- Ruka(I took pity and decided to give her a nice costume, but it does fit her personality)
80s guy- Shiki (I think the calm, cool, and collected gave it away)
Skeleton- Zero (well, I needed a dark costume for him, and now Kaname can tease him about "boners")
Angel/Devil- Ichijo(ah, my man defiantly is an angel and a devil (if you know what I mean) ok I'll stop)
Cop-Kain(He needed a costume that was him and somewhat stylish, because he's a nice guy)