I think I would probably mix the zinnias and the cosmos up in the center, and edge it (roughly, not in some kind of regimented way) with the wallflowers, personally. The cosmos are shorter, but they're fuller than zinnias - the zinnia flowers would stick up out of the lacy cosmos foliage and look really nice - zinnia flowers are gorgeous, but the stems are a little bit scraggly on their own. Cosmos foliage would green it up a little. And wallflowers look fine on their own, as a kind of edging, in my opinion. Sounds really pretty and cheerful!
Thanks guys! Hykue - I hadn't thought of mixing up the cosmos and the zinnia, but now that you've mentioned it it sounds lovely. I'm really excited about the berry colors of the zinnia, and you're right, without something interspersed with them they'll be scraggly. Putting the white with them sounds lovely! Do you think the shorter cosmos will get enough sun? (I am fearful of my black thumb for non-vegetables and have no idea how to space beyond what's on the back of the seed packet.)
I would stick some chives in the front. I love the purple flowers. Zinnas are great.Fills in things good until your perennials fill in. You could dig up those daffodils and move them to places you want them. I have some popping up in my far back yard that I will dig up when they are done blooming.
I am into edibles so I would stick some strawberries and ground cherrys in there too! Have fun.
I'm pretty sure the cosmos would be fine for sun - they have such finely divided leaves that they are good at catching sun, and the tallest parts of zinnias end up being mostly stem, so they don't block much sun. I haven't ever actually tried that combination, but I imagine it would work.
Chives are pretty nice too. For my color tastes, they wouldn't mix well with all the warm colors, but I'm a color segregationist when it comes to flowers. My plan is to have a bed laid out in rainbow fashion, with all the purples and blues together, all the pinks together, etc.
And yes, digging up the daffs to plant in better locations is a great idea - my parents did that with both daffodils and irises they found on their place, and now they have a lot of both! They actually ended up with so many irises that they started burning the extra rhizomes, because they didn't know what else to do with them. And then some of them sprouted right beside the burn pile! So I think you'll be fine once you get to the irises too, as long as you get varieties that do well where you are (and since you're already ready to plant, I think you've got a great climate for irises). In some ways I think they're actually easier than other plants, because they don't need as much water.
Anyway, have fun, I bet it will look great!