1. Does 'costume' usually refer to women's dress? We usually speak of women's clothes. The word costume is seldom used. It is most common at Halloween, a special day when children dress up as clowns, witches, vampires, etc. Such uncommon and special clothes are called fancy dress costumes.
Such uncommon and special clothes are called fancy dress costumes. Note that the "fancy dress" part of that phrase is British/Canadian -- in the U.S. you would just say, for instance, "What kind of costume are you wearing for Halloween? Last year, lots of kids wore Spiderman costumes."
In the U.S., a "fancy dress" is a dress that a woman would wear to