Researching the structures of each acro on aims database is how most learn about acropora identification and growth forms. That should help anyone wanting to know about this subject.
Mount them on a small piece of rubble, and then epoxy the rubble to your rock scape. That way you can still move them; even a year or two down the road.
More important than space is growth form.. Putting 3 branching corals even 6" away leaves them only few inches before the touch. If you add a tabling acro to a corner overhang then adding a staghorn or branching coral 4-6" behind is ok given one will grow toward the front panel and one should grow more toward the light