As to why Cordelia wasn't changed, I always thought that it was a sort of role reversal for her and Buffy, where the Slayer was the frightened, helpless, basically useless one where Cordelia, who had been, if not all of that in real life, was pretty close. As for the outfit, well, that was just the sort of thing that the (self acclaimed) hottest girl at Sunnydale High would be expected to wear.
What struck me as strange was, why didn't a single kid in Sunnydale want to dress up as Superman, Ninja Turtle, or a Western gunslinger? (The obvious answer, of course, would be that Ethan carefully eliminated all such costumes from his inventory, but just imagine if he had overlooked a few costumes)