Given the different note structure of a minor scale, do the typical rules that apply to progressions (in terms of sound, function and mood) apply?
Forgive me if the typical I ii, iii, IV, V, vi VII don't apply anymore due to the different shapes of the scale, but if so does iii still lead into V for example?
A lot of music uses the minor scale, it would be nice if this was clarified in the books (forgive me if it was, I didn't notice it)
Thanks for the question.
Chord progressions built around the minor scale have aspects that are both similar and dissimilar to their major counterparts. And this should make sense: on the one hand, much of the explanation of why certain chords go together centers on their bass note (the reason for instance that iii and I6 have similar function), which doesn't change when we change scales. On the other hand, the chords all have different quality (major, minor, diminished) and thus it's not unreasonable to expect that their relationships must also be different.
A detailed discussion of this topic is the subject of chapter 5 in Hooktheory II. In this chapter, you'll learn about relative chords, the idea that each chord in major has a corresponding chord in minor (for example, I6 in major corresponds to III6 in minor). The reason this is possible is that the minor scale is a special type of scale that is just a shifted version of the major scale – the minor scale is the major scale starting on its 6th scale degree – which is why it is sometimes referred to the "6th mode of the major scale". Because of this, corresponding chord progressions that sound good in one mode will also sound good in a different one: vi → iii6 → IV(add9) is a fine major progression, so we can expect that i → v6 → VI(add9) will work in minor.
All of this notwithstanding, major progressions also appear in minor songs in a parallel sense, that is, which scale degrees being preserved. For example iv → v → i (which you'll recognize from major) is valid in minor due mainly to the natural pull of the bass scale degrees 5 → 1, which is true even though the quality of the chords is different. Much more common, however, is mixing elements of the major mode into the minor mode, strengthening the weakness of the minor quality "five" chord: iv → V (maj) → i, where we say that V (maj) is "borrowed" from the major mode. This technique is known as "modal mixture" and is covered in chapter 6.3 of Hooktheory II.
Play around with these ideas in Hookpad and let me know what you think!