Here's my 3.141592 cents. This is a bit long and ranty, but hey, it's the internet.
The order in which different concepts developed historically is not necessarily the order in which they should be taught.
That said, scales predate and are more fundamental than chords and harmony, which should be obvious if you look at the history of Western music and then at other world music. All world music has rhythm. Pretty much all of it has scales. Only those styles derived from Western music have chords, and Western music first developed the idea of polyphony (which is essential to harmony) around A.D 900, which is really not that long ago. This quote from the Abbey of Regina Laudis makes that clear:
The simple, pure lines of Gregorian chant go back to the origins of the first Christian communities and the earliest recorded Western music. It is sometimes referred to as plainsong, because it predates the use of harmony or polyphony.
The order in which most people teach different concepts is not necessarily the order in which they should be taught.
Also although scales are more fundamental concept than are chords (melody predates harmony), it is still IMO true that to improvise in a purposeful way in harmonic music, your note choices need to be driven by the harmony, which are a subset of the notes in the key scale. You can always deviate from the chord tones if that's what you hear, but the chord tones (in a traditional jazz/classical context) are more musically definitive and therefore important than the non-chordal scale tones.
The way a concept was derived is not necessarily relevant to how it should be analyzed or used.
Modes. The whole concept of these is SO misunderstood and misused, it drives me crazy to read some of the discussions. The usual modes were/are just a convenient way to create some good-sounding scales that can be used.
The Major scale was one of the first scales put into wide use in Western music. The ancient Greeks, being the mathy and sciencey guys they were, attempted to get the most variety out of this scale without losing its defining structure of half/whole steps. So, they gave names to the scales based on each degree of the major scale. That's how they came to be defined.
But is that what they ARE? Is that what defines how they should be USED or TAUGHT? I don't think so. Don't compare the major scale to each mode derived from the same notes. Compare the major scale to each mode starting from the same ROOT. This will give you a clearer picture of how they function musically. For example, the Mixolydian scale is NOT at its core a scale that starts on the 5th degree of the major scale (although that is how it was "discovered"). Rather, it is a major scale with a minor 7th degree.
So, starting from the major scale, we have (in CODE so formatting is preserved):
Code:
Ionian: Major scale
Dorian: Major scale, b3, b7
Phrygian: Major scale, b2, b3, b6, b7
Lydian: Major scale, #4
Mixolydian: Major scale, b7
Aeolian: Major scale, b3, b6, b7
Locrian: Major scale, b2, b3, b5, b6, b7
Now, you can compare all these modes to each other. The scale degree they started from doesn't matter. The sound of the scale, that's what matters, and this is how it functions musically.
Here's each mode compared , showing the scale degrees on the corresponding 12 tones:
Code:
Ionian: R_2_34_5_6_7R
Dorian: R_23_4_5_67_R
Phrygian: R2_3_4_56_7_R
Lydian: R_2_3_45_6_7R
Mixolydian: R_2_34_5_67_R
Aeolian: R_23_4_56_7_R
Locrian: R2_3_45_6_7_R
Viewed this way, it's clear how these scales interrelate and how they will sound relative to each other.