Just wanted to follow up regarding the so/sol pentatonic tone set: it is not that rare in American folk song. We have 21 songs using that scale on our American Folk Song Collection website (http://kodaly.hnu.edu) and more in the pipeline.
Pentatonic tunes in general don't necessarily lend themselves to standard tonic/dominant harmonizations, but like modal tunes, they have their own underlying structures that make harmonization possible, though these may well be more intuitive for a group of singers (as Jean notes) than on chord-based instruments.
Sol-based tunes (mixolydian family) often imply what might be called the "subdominant as dominant," that is, a basic I - IV pattern, going all the way back to Mode 8 of the medieval system ("Hypomixolydian") which has the fourth scale degree as the "dominant" or reciting tone.