For the i-iv-v-i progression of a minor, do I use the melodic notes or harmonic notes?
Harmonic and melodic minor aren't really keys unto themselves. They are variations, or substitutions of the natural minor key they are based on. Which you use ultimately depends on context and intent. Many minor pieces will use all three at one point or another.
For i-iv-v-i your melody would use notes from natural minor. The v-i cadence is rather weak though so if you need a stronger resolution you can raise the seventh degree to get V-i. This is harmonic minor and its purpose is essentially just to borrow the leading tone (half step below the root) from major to strengthen the cadence.
Playing the harmonic minor scale, however, can sound a bit jarring and exotic due to the minor 3rd gap between the minor sixth and the major seventh. We can solve this problem and smooth things out a bit by also raising the sixth degree, giving us melodic minor, but by doing so we must also change the progression to i-IV-V-i (unless, of course, we're descending).
So, in summary: on a i-iv-v-i progression you would play natural minor. For a stronger resolution you would change it harmonically to i-iv-V-i and adjust the melody accordingly. If this throws the melody off too much, you can adjust it melodically, raising the sixth degree and adjusting the harmony accordingly, to i-IV-V-i.
Which ever sounds best to you; it make take a bit of work to determine that. I would guess that, as all three primary triads are minor, one might use a natural minor in most cases. (With any chromatic passing or neighbor tones help things out.)
Classical composers have treated the minor mode as a single thing; they would use whichever scale pattern they found sounded good.
You use whichever you like! Read on if you wish to label your decision with a bit of 'theory'.
If you use i-iv-v-i, you're using the notes of the natural minor scale. If you want a stronger dominant-tonic in the last two chords, you can change the v to V. This uses notes of the Harmonic Minor scale (and gives a strong clue as to WHY it's called the HARMONIC minor!)
You might use the notes of the Melodic minor scale and use IV and V when the melody rises up to the tonic, v and iv when it falls.