i think of keys as being only major or minor, or else things can get muddled up.
for instance, i would think of sweet home as being in the key of G, and its using a V-IV-I progression.
from there you can figure out all kinds of cool things.
when i hear the word "mixolydian", i think V or V7 and thats it.
but for the sake of what you want to do....lets do a mixo "key" in Cmaj.
so, it would start on G.....and G would become I......and so on....so it would go...
I-ii-iii(dim)-IV-v-vi-VII.
Gmaj Am Bdim Cmaj Dm Em Fmaj
so, if you write some progressions out of that, youll probably find they are easier to think of in terms of major or minor.