In your warmer zone, most crapesmyrtles will be the size of small ornamental trees. And, mature about as wide as they are tall. You could try combining mid-height crapemyrtles and hibiscus in a hedge row and see how it turns out. Though, I think sticking with a row of hibiscus would be an easier hedge to care for. Then, add a crapemyrtle as a separate tree in your yard.
Thanks for writing!