I grew up in Los Angeles where everyone says "you guys" regardless of gender, even in an all-female environment. My parents always said "you folks" which was probably a more genteel alternative, and taught us children not to address adults as "you guys."

In college (also in southern California) there were a lot of kids from Texas. We teased them a little for saying "y'all" but admitted it was a little nicer sounding than "you guys." Funny -- they said "all y'all" when addressing larger groups, which tickled us.

One day we asked the kids from Texas how they distinguished between "y'all" and "all y'all." They thought for a moment and the consensus given was that for one to four persons, use "y'all" and for five or more, use "all y'all."