There are a lot of perennials that go well with grasses. Among my favorites are Coreopsis (like Moonbeam and Zagreb), Rudbeckia, Liatris Kobold, Purple Cone flowers (any of them), asters, Lavender and other grasses, of course.
Karl Foerster grows 3 feet wide, I believe, so I would plant it 4 or 5 feet from the wall, you will want to leave some breathing room behind it.
I have planted grasses in the fall with good results, but I have read that for more success they should be planted in the spring.

"anitamo has suggested a few very good perennials to go with grasses but there are so many more that can be plugged in. It all comes down to the look you want and what is yours or whomever's garden style you are developing.

As for "Karl Foerster" assuming it is Calamagrostis acutiflora "Karl Foerster", ours are now 3' wide and they are 6 yrs old. They need to be split badly. "Karl" is a very vertical grass, yes some have had "Karl" arch outward but I believe that is not normal for Calamagrostis acutiflora "Karl Foerster".

Now if you are growing Molinia caerulea "Karl Foerster" then you will have and arching "Karl" that will get 3 - 5' wide as it arches and 7Â high in flower.

Now the planting of grasses is a debatable subject. It has been read and stated plant cool season grasses in the spring and fall while warm season grasses are to be planted in the summer.

I on the other hand have planted grasses whenever I got them regardless if they were cool or warm season grasses or what season of the growing cycle it was at the time I planted them.

As for planting location, "anitamo" is right in the ball park that I play in. Always give yourself at least 2Â feet clearance from any structure, wall or building from a full grown plant.

Plant grows 5Â diameter then plant it 5Â from the wall.

Jake

At a display garden today I saw an enormous Miscanthus 'malepartus' somewhat screened by a huge clump of Fennel and a very large New Zealand Flax (Phormium). Very striking grouping with terrific texture.

The Perennial suggestions are very good. I also very much like the way Pennesitum Orientale lookes with larger miscanthus. Right now I have a Miscanthus 'Arabesque' in front of a huge 'Morning Light' Miscanthus and I'll be replacing the 'Arabesque' next season with the Pennesitum (unless someone can suggest a full sun (HOT) tolerant plant that they think looks incredible paired with the 'Morning Light.'

Karl Forester suggestions are solid. Be sure to let these grasses have adequate air circulation. They can develop rust. Consider also the way a drift of 'Morning Light' Miscanthus would look against the brick? I love that grass...

General opinion is fall planting is ok, spring is best. Sometimes it's hard not to just blam the darn plants in whenever. Haste can make waste though!