If you would be willing to plant a tree with mildly allergenic pollen, but still at your 20-foot height limit, then you could include many fruit trees among your choices, such as citrus ...

Many landscapes rely on a lawn and large shade trees, but the addition of medium-size trees provides a transition between the lawn or groundcovers and the shade trees, creating a complex ...

the small and medium trees link the tall shade trees to the shrubs, perennials, ground covers and lawn grass. Small and medium-size trees also often mark the formal border between property lines ...