Susan would be a cherry pie! Because cherries are red and expensive, and sour cherries are usually used in pies. (Not that Susan's sour, but she's not exactly sweet either...)
Romilda is going to kill me for this, but she would totes be a pork pie. Pork pies are "traditional British food" according to Wikipedia, but they're also popular in Germany, which reflects Rom's dual heritage. I also like the play on words, as Romilda's a bit porky herself, and she likes telling porky-pies to get attention.
I'm going to be really biased, and say Draco is a fish pie because they're both disgusting and leave a bad taste in your mouth...