Answer: Erythrina falcata, the coral tree, might suit the site. It has an open branching habit and airy canopy. Cotinus coggygria, also called smoke bush, has an airy look, as well. My absolute favorite is Albizia (silk tree) with feathery foliage and puff-ball looking flowers. It gets a bit taller than your requirements, but would be a nice complement to your other plants.
Hope one of these suggestions appeals to you!
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