Well it's probably a whole different feel jumping from slayer to country. Its called the POCKET!! Haha. I know where you're coming from. I grew up playing punk and prog and my first blues gig was a nightmare!! It's sounds soooo simple. Until you have to settle into that groove and you realize how every millisecond of each note affects the groove.
As far as scales go..... A lot of major pentatonic, minor pentatonic, and a lot of passing tones. It depends on the situation. When a song only involves two or three chords you have to be very creative in your approach to those chords. Theres a ton different ways to make it happen. Chromatic approaches, chord tones (arpeggios), and different rhythmic approaches. The up beats and and down beats can add different feels along with what note you choose to approach the next chord with.
Everybody seems to think country music is soooo easy from just listening. But it's definitely an art unto itself. Like boxing..... You have so little weapons to use, but there's sooooo many different ways to use them. And it's an art figuring out how you're gonna make that jab come from thirty different angles!!