A comment discussion under this question from me, clued me in to the fact that I don't really understand how minor keys work very well. So here is what I think I know
The natural minor comes from the Aeolian mode of the major scale. It gives us the following triads for use in harmony:
i ii° III iv v VI VII
From what I have been told, classical minor harmony also utilizes the harmonic minor. which is the same scale with a sharpened 7th. This implies the following triads (notated with respect to the degrees of the natural minor):
i ii° III+ iv V VI #vii°
Now the comment "the 6th and the 7th scale degrees can be modified whenever the composer feels it's necessary to do so." leads me to believe that this is not an all or nothing thing, and I can more or less mix and match to suit the sound I am looking for. First of all, am I right up to this point? Should I tend to favor the triads from the harmonic minor?
Taking this further, I could imagine different combinations of sharpening or flattening the 6th and 7th scale degrees. Well there would only be 3 other choices really: Melodic minor, a #6, or a #6 with a b7. These will each imply different triads. Are any of these (besides melodic minor) going to be harmonically useful? Can I effectively use any of the resulting triads in a progression and not sound out of key? I realize this may be subjective to a certain degree.
To throw another wrench into the gears, how do other "minor" modes play into this? I am considering a scale mode to be "minor" if its 3rd is a minor third and the 5th is a perfect fifth. For example, in the dorian mode: Can I play the same tricks with the 6th and 7th there too? Or will I run into problems?