i always thought you just had the shape of the scale and just changed the root note for a different scale, like Cmaj starts on the 8th fret and Bmaj would start on hte 7th fret. so i would think Am would just be a minor scale starting on the 5th fret. the Am scale would be (starting on the low e string) 5-7-8 5-7-8 5-7 4-5-6 5-6-8 5-7-8. so for the Am scale a I-III-V-I chord progression would be A C E A i think. i dont know exactly, so this was probably a useless post but you can try it out and see how it sounds cause i may be right.