I found, by trial, and lots of error, that "mixed beds" don't work for me (I know lots do it with success....but I can't). and I think the following reasons are why....when the "other" plants need water...the iris don't, and when they need some "nitrogen boost....the iris don't....the iris need the sun on the rhizome, or at least the soil OVER the rhizome, to keep it dry.....and the "other" plants block the sun.....the iris also need good airflow around them, to help with drying, and fungal diseases...the "other " plants block this airflow.....without rigorous deadheading, the "other" plants will 'self seed' into the iris clumps....I have enough of that already, with blown-in weed and grass seeds. There are a "few" plants that I "allow" in some of the iris beds........the spring bulbs...tulips, daffies, and such, because the foliage dies back quickly enough to not cause any of the aforementioned problems, glads (another irid, really!), because their vertical growth doesn't either shade very much, or block much airflow, and true lilies....again, vertical growth.
You will have to find the "system" that works for you, in your climate, and conditions....if you want"mixed beds, I would say increasing the space between the iris to several feet, and not crowding them any closer than 10"-12" with any other plant.
