Mary, iris works very nicely with peonies. But dahlias and lilies, not too well for me. They look kinda strange because the lilies and dahlias esp. the tall ones are so much taller than the mound shape of peonies so they sorta stick out. I think they can work but you have to place them strategically so that the difference in height does not detract from the bed but enhance the whole effect. Easy said than done though. Plants placement has been a hit and miss thing for me. Plants that combine well with peonies for me are iris, clematis, hardy geraniums, and daylilies. Grass is also very nice at hiding dying peony foliage in the fall.
