Is there some reason why people seem to assume the Supervision Army is just a remnant fighting for it's life? (Other than the fact they have not shown up on screen). Because I get the impression from the way they are discussed in SDFM that they are a significant force, possibly a match for the Zentradi.
I mean, Bodolzaa's fleet was the 118th Zentradi Fleet and had 4.8 million ships, and his reason for wanting the Reaction Weapon technology was to gain an advantage over the Supervision Army. (I can't remember the exact episode where he said this, but he did, I watched the whole series again last weekend). If a warrior civilisation with a total force in excess of half a billion ships is desperate enough to risk contact with miclones (which they believe could potentially cause their downfall) to gain an advantage in their war, then it stands to reason that their enemy must be formidable.
As to why they have not been encountered, in episode one, Britai mentions that the Supervison Army vacated the region around Earth 8 terms ago (how long is a term by the way?). Who know how big this region he refers to is. It could be that the war is concentrated in other parts of the galaxy and 50 years and 55 fleets does not give enough time or coverage to encounter them.
I don't think Macross 7 contradicts this. I haven't watched it for a while though.