I'm not familar with that hydrangea but it sounds wonderful. And I think the red 'Fanal' Astilbe would be pretty in between them with the white Impatiens in front.

I think Campanula persicifolia (peach-leafed) may do well in your zone and fit in?

All my hydrangeas are "Forever Pink"--very nice dwarf shrubs. Of course, something should be said for diversity but heck, they were a deal :-)

I think a pleasant arrangement would be to place the six hydrangeas along the length of your beds and kind of toward the back, plant the astlibe forward of the hydrangeas and place them so they are staggered (the astlibe would be forward of the space between the hydrangeas), then fill in the front gaps with white impatiens. That would create a very symmetrical, formal bed.

Personally I grow calla lilies with my hydranges but astlibe will probably be a very nice contrast. One thought, what will the winter interest be?