Alliums are better off being planted amongst herbaceous perennials, rather than shrubs as shrubs can quickly become large and widespread. Perennial grasses would be ideal for growing with Alliums. You could try Stipa tenuissima or a variety of Carex. Hardy Geraniums also make good partners, with their thick clumps of leaves appearing in spring. Sedum spectabile produces thick foliage which would disguise the Allium leaves along with Alchemilla mollis (Ladys Mantle). Alchemilla mollis has bright green/yellow flowers and makes a superb contrast with purple Alliums. There are many varieties to experiment with.