Considering: that finding the Ionian Mode's Phrygian location, suggests counting back 3 degrees from C'- Bb, Ab, 'G', starting from 'G'- play the 'Eb' Scale. The Answer', I have been serching for the last 18 months is: IF' I am playing the Eb' Scale from G;, and others, are playing the Ionian 3rd, E'Phrygian, is the Tonality between the 2 compatible?
No. G Phrygian uses all the notes from the parent scale of Eb. E Phrygian uses the notes from the parent scale C. They are not the same set of notes. I suspect you meant:
'If I play the Eb scale, and start on G (the 3rd, therefore G Phrygian), if others are playing the Ionian of Eb, are we all using the same notes?' Then, since Eb is the parent, then - yes!
Eb Ionian = F Dorian = G Phrygian = Ab Lydian = Bb Mixolydian = C Aeolian = D Locrian. As far as notes contained therein are concerned.