Why not mix? A row of several similar plants sounds like a good idea until you loose one and need a replacement. Mixing it up means you can fill an empty spot or replace a loss without worrying about matching anything.

Whatever you choose, I do think it would be a shame to have such a lovely wall and not have something spilling over the edge of it. Creeping phlox, calibrachoa, trailing petunias, any number of things but something.

I would tend to throw in some broadleaved evergreens or conifers so winter isn't so bare.I have some very plain Blue Rug Junipers spilling over a retaining wall and the effect is gorgeous.

I'd move the Macs 12-18" toward the wall and then stagger Oakleafs or arborescens behind. If you choose Oakleafs consider sticking with ones that grow 5-6' in diameter and not the 8-12 footers.