You are a zone warmer but in my experience you would be fine for the Peony but not the daylilies.

If I was a better spitter I could spit into zone 5, the boundary is so close, so your advice is perfect!

I have a peony behind and off to the side of a planting of 3 large zebra grasses. The peony does just fine in terms of bloom. I think what also helps is this is a north location but is far enough away from the house that they get the mid-day southern sun -- so in essence the peony is actually in front of the grasses. One thing I have noticed is this particular peony gets mildew pretty badly by the end of the season. I never struggled with mildew on peonies before this, and IDK if it's less than optimal air circulation later in the season or what. This is something you should also consider -- can you live with the mildew? It doesn't bother me because the peony isn't conspicuous after the grasses fully leaf (blade?) out and the other stuff other stuff in the general vicinity starts blooming, which takes my eye off the peony.