Peter T., you seem to have gotten the relationships among the scales down. You can if you wish define a species of dorian mode with a "dominant" on A, and some treatments do this, but not every air in the dorian mode necessarily has a dominant on the 5th.

I repeat my preference for avoiding seven chords as tending to sound out-of-place in a dorian or mixolydian context.

M.Ted, your treatment is a sort of counterpoint theory I have never encountered. (Well, to be honest, I haven't encountered much counterpoint theory of any sort.) You seem to be saying that in some circumstances you can play against C-major melodies with G-mixolydian and D-dorian countermelodies. The overall context of your description seems to be a chromatic major-key context, which wouldn't seem precisely to answer Peter T's original question of how to accompany mixolydian and dorian melodies in a way that reinforces the unique characteristics of each mode--though your comments are certainly fascinating in their own right, and I hope you'll elaborate on them some.

I was familiar with the terms "tonic" and "dominant" referring to positions in a particular scale. In your earlier post you seemed to be using them as absolute characteristics of entire scales. Now I see that you mean them as relative terms: the G-mixolydian scale is the "dominant scale" to the C-major scale. Presumably the D-mixolydian scale is the "dominant scale" to the G-major scale in your terminology. Is that right ?

T.