Recently I picked up modes for guitar scales. I just love how this system works and understand it 100%. However, I have heard about other scales that are not covered with the modes. More specifically, I heard that there are a melodic and a harmonic minor scale. Can anyone tell me the difference between them and between them and the 7 modal scales is? When do I use them? Are there even other scales being used outside of the 7 modal scales?
There are already a lot of answers pertaining to this question on this site. However, briefly, the harmonic minor uses the notes from its relative major, with one change. In A minor, for example, the notes of C major are used, with a change of G going to G#. This is because the leading note sounds like it's leading better from one semitone below the tonic. Left as it was, it would be a tone away, as it is in the natural minor.
The melodic is a different kettle of fish. Again, based on A minor,(or any other minor...) the notes rising are identical for the first five. Then it gets both a sharpened 6th and 7th. However, in the classical version, all the notes descending are the same as the natural minor. Modern jazzers tend to use the rising notes mostly in preference to the classical version.
There are modes worked from each degree of these scales, which produce some interesting sounds and new chords.
As far as other scales are concerned, a 5 second search via google gave a plethora of very diverse scales, as well as the main standard ones we're used to.
If by modal scales you are referring to the modes of the major scale Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian and Locrian, you should know that the same principle applies to the minor scales. You can think of the 7 modes as the same scale, but starting in different notes. If you play C major in a keyboard, it's all white keys (that's the Ionian mode). Then play D dorian, again all white keys; E phrygian, etc all the same.
So you can do the same with the minor scales; for example the melodic minor; it's usually considered a minor scale with raised 6th and 7th degrees, but you can also look at it as a Dorian mode with a minor 7th, or an Ionian mode with minor 3rd, even if that's not strictly correct.
The other six modes of the minor scale are Dorian with a minor 2nd, Lydian with an augmented 6th, Lydian with a minor 7th, Mixolydian with a diminished 6th, Locrian with a major 2nd, and Locrian with a diminished 4th.