Keep in mind that they will not grow to a specific predictable size and then remain frozen there; the sizes reached by trees and shrubs varies with how they are handled before planting and with how they respond to the specific conditions of a planting site. Trees and shrubs continue to grow larger until they enter a decline and die back or die. Viable specimens make new grow every year.

Note also that Amelanchier canadensis is a loosely applied name in commerce with more than one other species being traded under the name. The true item is a vigorously suckering, comparatively short-growing species. Other species sold in its place tend to be more arborescent (tree-like).

Most serviceberries are shrubby trees and may sucker or grow bushy under conditions promoting this habit more than a tall one. And some stock may be grafted on a different, more suckerous species than the one purchased (as a scion). For example, a 'Cumulus' I planted at a friend's place decades ago has grown into a tree maybe 20' tall with a clearly defined single trunk - fringed at the bottom by suckers of what appears to be western serviceberry rootstock that have never given up trying to form their own top.

The viburnum, as you say can be grown as a small tree if trained into that shape. Otherwise it produces a typical viburnum clump of angular branches.

If size is a concern select plants that do not grow into small trees or large shrubs. Stay completely out of that size group and you will not have to worry about something becoming 30' instead of 20' within your time on the site.

Even A. arborea grows in multiple branches. I think that A. canadensis and A. arborea have the same form and structure, but A. canadensis should, as you say, be shorter.

You have a good point with regard to the suckering. The Amelanchier canadensis I have are probably grown from seed - they're just the straight species which are common in my area.

However, the Alemanchier x grandiflora ("Autumn Brilliance") could have been grafted on anything, although likely an A. canadensis. In time, I think the 2 trunks that it came in the form of will die away and be replaced by suckers (the variety of the root-stock). Since I don't feel like doing a life time of grafts, removing scion wood from the 2 established trunks to be applied to the suckers at the base, I think that I will forget about keeping the "Autumn Brilliance".

I could take seeds from the "Autumn Brilliance" and end up with a specimen which contains genes from A. laevis and A. canadensis (the two which make up "Autumn Brilliance"). This way I can be sure that my specimin always stays a a. laevis and A. canadensis hybrid. I only want these two because they are native to my state/county. Would the progeny of "Autumn Brilliance" (even if pollinated by A. canadensis) turn out to be a hybrid like the parent (A. laevis + A. canadensis)?