I've never heard 'mate' used for a woman, but I can imagine it's possible. I consider it totally normal to call a group of guys and girls together 'guys', as in 'what are we going to do then, guys?', but when I've used this with foreigners, the girls find it a bit strange! In Glasgow, men used to use 'doll' to address a woman in an informal way, but I know at least one woman who also uses this word to address a guy - it is slightly strange, but it just comes across as good-natured and friendly.