Blue sea holly with russian sage?

The Perovskia (sage) is as tough as old boots when it is established and if it likes it will take over an area you have described. you need to keep a close eye on the roots as they send runners everywhere. As suggested below, Perovskia is not an herbaceous perennial but is a shrub.

Echinops ritro might suit at the back of the bed, and that lovely small grass Festuca glauca at the front.Artemesia 'Powis Castle is another idea. dotted around.

A good filler could be a spread of Elymus , the blue rye-grass, just keep it under control.

To break up the silver effect try Gladiolus byzantinus.

Convolvulus cneorum could be good at the front of the bed as it can trail over the edges effectively.

Sea holly has deep tap roots and re seeds itself each year. It dies down to the ground & dies each season an the next spring seedling sprout up at random areas of the garden.

Russian sage is a perennial dies to the ground each fall. It does not move well during the growing season. After the plant has flowered trim off blooms then it can be dug and moved to other areas of the garden or moved in early spring before the temps warm.

Both plants grow to about the same height as the sage is wider and sea holly straight and a pleasant companion plants for the garden as the both require the same growing conditions and with wispy sage in the background, the sea holly will be the star.

I have planted evergreen heaths and heathers which add color to the garden all year. They bloom at different times of the year. Other companion plants petunias, snapdragons, lantana, cosmos, zinnias, roses, marigolds, teddy bear sunflowers, Lilly's, dahlias, as well as small shrubs mini spirias with lime green foliage grow to a foot tall . To see pics of the many variations of flowers & plants that can be paired with UR garden see plant companions.

See links below for pics and growing guides.

https://secure.bhg.com/common/profile/regStep1.jsp...

http://www.youcanlearnseries.com/Landscape/Plants/...