I suspect that the tall variety you have is hardy ageratum, Eupatorium coelestinum, and its arrival in your yard doesn't have anything to do with having planted annual ageratum. In my garden hardy ageratum spreads/reseeds some, but the excess is easy to remove. I see complaints from deep South gardeners about its invasiveness, so apparently it can behave more aggressively there.

I love this flower. I planted some early last fall that have spread about a foot in each direction so far. But it is not dense, overwhelming growth and does not seem to pose a problem for neighboring plants (yet). The plants are very, very shallow rooted and are easily uprooted even when not really trying to do so (i.e., pulling adjacent weeds). They were very late to emerge and at first I thought they were dead until I saw some growths emerging from the soil in early June. The color sparkles and shines in the subdued autumn light.

I haven't tried that one, but Ageratum houstonianum (Floss Flower) did fairy well back in my Eastern WA gardens. Here is a link that might be useful: Floss Flower

I agree completely re the "looser look". In past years, there were way too many plants :(. This particular variety of hydrangea will get bigger but it is advertised as less than 2 feet. This particular salvia is so very good. It doesn't get too big. And it blooms *forever*. The only one I had last year, planted in a container, was punching out blooms past mid October. Only a killer frost will bring it down.