Hop bush is a great suggestion (Dodonea viscosa and Dodonea viscosa 'purpurea'- the purple tends to come out when chilled - it's also a bit more frost sensitive, but next to a western wall it should be fine). I also love the Fern of the Desert (also called Fern tree, Fern bush, etc. - Lysiloma thornberi). You can also get the soft, lacy look from Guajillo. They are all very drought tolerant. If you want some pretty blooms and a bright green leaf, I would suggest the desert willow, Chilopsis linearis. It has the added benefit of being deciduous, so that same western wall can warm your home in the winter. I went down to Baker's last spring and looked at some of their trees, with the same parameters in mind - a number o the Eucalyptus are small, with yellow or red blooms. I bought a mulga - it has silvery leaves and bright yellow blooms that look just like catepillars. It grows to about 15 feet. Lemon Euc and Forman's Euc are two more you might want to look at. There's tons more, we're lucky to have a large number of smaller trees. Don't forget the Texas Mountain Laurel (both green and silver leaf forms) many of the Acacias are smaller trees, the Screwbean Mesquite has very interesting seed pods, Caesalpinia cacalaco, mexicana & gilliesii are all very pretty, blooming trees. HTH!

Schefflera pueckleri does develop some rather aggressive surface roots with age, and may be perfectly safe it your wall has a properly reinforced foundation of concrete with rebar, but not enough info here to advise you in your circumstances. In general Podocarpus latifolius or another species is probably a safer choice if your retaining walls are of questionable construction.I've also seen Magnolia grandiflora dwarfer cultivars used as trained espaliers in narrow planters without foundation problems. As to just one 2 month drop of flowers for Bougainvillea, not how I see them work here in northern California where most cultivars have two to three waves of bloom throughout the year, and drop massive amounts of flowers. Not a vine I'd use by a pool without a pool cover.

In book "Wild Flowers of Florida", I found an explanation for the difference(s) between my tree and photos /descriptions of B. halimfolia. On individual trees, all flowers are either male or female. Author wrote of male flowers that they are "much less noticeable". Perhaps foliage density tends to follow suit. I am prompted by that reading to correct the above-stated "Deciduous" to evergreen. (Leaves did leave; now I recall that I pruned the branches.)

They don't tend to grow much higher than a one storey house. You cazn also consider dogwoods (many are native), Lewisia (get the native one) and nativee viburnums. Fedco has a bunch. I presume they ship to Canada. Their prices are good and quality high. The size shipped tends to be small, whichnis great for me, it's far, far easier to plant a smnall, bare-root tree in my rocky, rooty soil than a big, potted shrub.