Cupcake Cat
2. Why do people think Envy is a girl. Can't a person crossdress?
Well, apparently Envy couldn't remember his true form, or something like that. And he isn't actually a male or female. But one usually uses male pronouns when a character's gender is ambiguous. Like with Mokona and Ashura from Tsubasa--CLAMP purposely decided they weren't to have actual genders, but when pronouns were absolutely necessary the pronouns were male.
So, that's why some people think of Envy as female instead of male--because it could really be either. But yeah, he really is more of a male character than a female one anyways most of the time.