Just have to ask ... What are then names of those Asters?

Such a wonderful combination.

Think I could do that in a container?

I don't know if that would work in a container, with these 3 asters, they are about 4 feet tall. They are quite large, but there should be some smaller type asters that would work.
How is your Sedum 'Autumn Joy' doing? I don't remember how big the container is that you put it in?
Aromatic Aster (Symphyotrichum oblongifolium 'Raydon's Favorite')
New England Aster (Symphyotrichum novae-angliae 'Andenken an Alma Pötschke')
Bushy Aster (Symphyotrichum dumosum 'Wood's Pink')
Bushy Aster (Symphyotrichum dumosum 'Wood's Purple')
Aromatic Aster (Symphyotrichum oblongifolium)

Great article about asters.
http://www.telegraph.co.uk/gar...

Nice list of some asters. But you have to sort through them for the shorter varieties.
http://www.diggingdog.com/page...

Give Timberline a call and see which ones they will be carrying this year. Bet they have some nice one.

There are 10 pages of mostly asters to browse through. They are such easy amazing plants that come back every year. Unless you have a garden helper that pulls them all up and disposes of them while you are in the house fixing lunch.
I found two tiny planst the end of fall, one is Aster Novi-Belgii 'Days' and the other is Aster 'Believer'. Now to see what they will do this year. I have not seen them in bloom.
http://www.lucasgreenhouses.co...
http://www.plantdelights.com/s...

OK now I have some homework to do. Will work on it.

The container is about 18 inches wide and maybe 18-20 inches deep.

4 feet would overpower it. I think.

Yes, but just thing how beautiful those low growing ones would be around the base of your Sedum 'Autumn Joy'. And they want full sun just like the sedum. What a great combo that would be.
Let me know what you find.