Next to reatining wall means different things. Their roots can wind up in weird places, usually sending up suckers, so no if it's two feet, but 10 feet, no problem, but then dwarfs may act differently then standards. Yes they enjoy the sun.

Here is a link that might be useful: Propagating Perennials

Thanks for the quick response.

But how do the roots grow? Do they spread out like a forsythia (I hate this plant, it should be classified as a weed!) would or down like a lilac bush?

While some plants can be said to have deep roots and some shallow, most plants - be they trees or shrubs - grow roots in the top 18-24 inches of soil, out to their drip-line and beyond. How far beyond is a matter of the species, the variety, the conditions under which it is growing - soil, water availability, etc. - but generally, you will get feeder roots out as far again. Some trees do have roots that can run from one side of a lawn to the other, but they are exceptions.

I have no specific knowledge of how CM roots grow, but would expect that it was more down than out, within reason - the larger the CM, the more the root spread to be expected.

Since CMs grow more up than out, and since their suckering is usually a matter of new shoots coming up from the base of the plant and not from roots many feet away in the lawn, I think, as long as you are planting at a distance of 3-4' way from the retaining wall, you should be fine planting one of the smaller CMs in the bed. One of the large, older CM varieties might be more problematic, but since you say you want a dwarf or small one, I would go for it.

If you are REALLY worried about the stability of the retaining wall, then I would avoid all trees or shrubs in the planting bed, and stick with perennials - 99% of them don't have roots that are invasive.