I probably wouldn't be too happy if I were alone and someone referred to me as a 'guy'. However, I have been guilty in the past of saying things like, "Guys! Hurry up and let's go!" my group of girl friends, and no one has ever got mad at me, so I think that it's only when there is a single female would it be impolite. :-)

Yes, you'd never refer to a single woman as a "guy" but in a mixed group it's definitely fine to use "guys" for the group collectively.

Only speaking for myself, but while I personally don't see a female on her own as a "guy", if it was a group of men and women I would have no problems calling the whole group, "the guys".

Oh, thank you very much! I'm satisfied with everyone's answer :)