I love your garden! Can I add to the above excellent lists? Achillias, some lovely colours about these days. And Scabiosa, again, some fab cultivars available in several different heights and colours. Lastly, Knautia, which is similar to a Scabious, available in a bigger colour range now. All of these are 'airy' plants which will compliment, but not dominate your grasses. They all have pretty good seedheads that stand the Winter well. Have a google!