Beginner Theory

Hey guys,

I am extremely new to theory and I've been trying to figure out how to determine what mode to use for chord progressions. Now, I know this is a HUGE topic and we can really get deep so I want to make it a little more specific.

Let's take the song "Hurt" - Johnny Cash version ( Don't worry I'm not out to bastardize the song, I'm just trying to apply theory to a specific instance)

Basic chord progression is Am, C, D, Am. Now if we wanted to create a little solo there I'm wondering what appropriate scales to use would be.

I've broken down the chords and put together that the Key is in G. (At least I hope I got that right...)

Basic understanding tells me an A minor pentatonic would fit. Is it also safe to assume staying in the G major scale works as well?

Secondly, what modes, and more importantly why, would fit in there? And would we change up the modes to match within the progression itself?

Thanks in advance.
-Matt