Daylilies should be happy there -- though you'd probably prefer something with foliage more unlike iris.

I am assuming you mean something other than bearded iris, as they don't like excessive wet of any kind or at any time. And you don't give any information on sun/shade conditions, which could have a big bearing on plant selection. What about irrigation in the summer months - yes/no? Winter rains in the PNW can be heavy enough to discourage a lot of more droughty type plants yet it is dry enough here in summer to create lack of water issues without proper irrigation.

Not sure why you picked the landscape design forum - this is not really a design issue. What is "medium height"? Do you want just perennials? You could try asking there (Perennials forum) with more detail or in your regional Northwest Gardening forum, again with sufficient detail to engender meaningful responses.

There is a thread in the Iris forum on this topic; you might want to check that forum for more suggestions.

That being said, bearded irises are mixed in with many of my beds, playing nicely with daylilies, roses, nepeta, montauk daisy, hary geranium, peonies, lilies, Japanese anemone, coreopsis, kalimeris, sedum, pansies, garden phlox, penstemon ... some small shrubs and other things I can't remember just now. As long as the soil is well drained, they seem to get along just fine.

Here is a link that might be useful: Companion Planting for Iris