Country is dirt simple so the basic I IV V with melody notes from the major scale will always work. But, music is not that cut and dryed, sometime Country is not dirt simple so that changes things, like when a vi or a ii get thrown into the mix for some color. Do we ignore these minor chords or do we incorporate them into our melody? If we went by a list of scales that work over a list of styles we may.
Pentatonic KHANcepts
http://www.stevekhan.com/books.htm opened quite a few doors for me. Here is an example of what is in the book.
Over a Fmaj7 chord (F-A-C-E) your choices could be:
Modes/Scales
F Major (F-G-A-Bb-C-D-E-F or F Lydian F-G-A-B-C-D-E-F
Pentatonic Options
Am pentatonic (A-C-D-E-G)
Em pentatonic (E-G-A-B-D)
Dm pentatonic (D-F-G-A-C)
Notice each of those share at least two notes with the Fmaj7 chord. That's enough to achieve harmonization.
Now if that was Fm7 then ...........
Mode/Scales
F natural minor or F Dorian
(then I add or F harmonic minor or F minor whatever..... like F Phrygian...if the other chords led you there.)
Pentatonic Options
Fm pentatonic
Cm pentatonic
Gm pentatonic
Look up the notes in all this yourself. That fish thing.
Are there more options we could pick? Of course.
Why those? You end up with like notes in the chord and in the melody. When that happens both the melody line and the chord line harmonize each other. I think that is what you are looking for. - Why not look to what chords are being used and let that govern what scale we play over them.
