Thanks for the quick reply Dietz. I just edited above to say
"to homogenise those beds with an "overall space" and spot 3 or 4 instruments as ambisonics/atmos objects and send from those to MIR. Is this approach possible or flawed?"
Yes I see what you are saying. More object orientated.
edit: what if my groups are 7.1.4, or anything greater than mono or stereo?