*blush* ok I made a mistake. In the key of C, adding an F to the G chord isn't mixolydian.
However, in reply to your message, a 'seventh' chord on the tonic surely has something to do with the mixolydian?
As you can tell, I'm not an expert, but isn't the whole flavour of the mixolydian mode the fact that it has a flat 7th?
I take you point - I guess that having seen loads of Americans here always posting the dominant with an added 7th made me assume that it was something of a national trait! Sorry for making stereotypes.
Interesting that you say bluegrass never uses them - will go back and listen more closely