The etymology of "guy" is worth looking up. Apparently derived from Guy Fawkes, who was burnt in effigy. And said effigies became a synonym for "grotesque person" which in turn became just "person". In view of that, male or female shouldn't really matter and, yes, I've heard it used for both genders. (Quite often in the "you guys" form, which is yet another way to compensate for a lack of appropriate grammar in English).

English did have a plural you, which IIRC was the word "you" and "thou" was singular (might be wrong here).

"you" was the formal form of "thou". In Ireland we use "ye" as the plural of you.