Beautiful, appealing, "kick-ass" even can be pretty subjective. There are those who love the Thomas Kinkade type jumble of everything always in bloom and at the same time in a riot of color that not even nature could conceive; and there are those who find every square inch planted up a tedious lot of work, while some like a little more order in their chaos.

What I would do is go out and move the plants in their pots around taking into account your yard's sun/shade conditions, the sun needs and shade tolerances, and spacing guidelines of the plants you purchased. Tall plants in the back so they don't obscure the shorter. Stare at the pot arrangement and do some shuffling. Beyond that, I can't think of a magic formula for an endeavor even experienced gardeners often get wrong - and tinker around with for years to get closer to right.

Shades of purple, blue, and lavender usually blend; can be very nice with a backdrop of white like the shastas. The soft yellow of the Happy Returns and the purplish maroon of the Pardon Me wouldn't seem to clash with the scheme. I tend to think - and maybe I'm way off base - but in a cottage style garden sometimes it doesn't matter.

Don't know which of the hundreds of varieties of wormwood (artemesia) you got, but some - A. Absinthium, I think - self seeds, puts out root runners, and spreads all over.

Would seem you're into the first frost dates for your region; perhaps even the first killing frost might not be too far off. You might get things in the ground and tweak in the spring.

There is a Cottage Garden forum here also.

"a kick-ass backyard next summer" is probably asking too much but who knows. If the ground is prepared and this collection was really cheap get them in the ground and sort it later, there is nothing earth shattering there anyway.

What duluth said.

6 hours of sun is considered full sun.

Coral bells (heuchera) can handle an almost complete lack of direct sun. They will bloom and be happy.

I have Happy Returns on the east side of a 2-story house -- morning sun only -- now being shaded by a lilac and a fig tree (I really will move the daylilies next spring ... I promise!). My Happy Returns would be happier with more sun, but blooms anyway, off and on all summer. In a couple of years, you'll have three or four times as much. Wonderful plant! [Daylilies don't grow true from seed, so don't let it go to seed unless you like surprises. Even if you do like surprises, it's probably best to start the seeds elsewhere, because if a seed sprouts in the middle of an established clump, you may not like the look of an anemic pink in the middle of the yellow. Some daylily seedlings are not worth keeping.]

I haven't grown Pardon Me, but it's the same "everblooming" type.

Because of the late planting, mulch everything well so they'll have a better chance of surviving the winter.

duluth didn't warn you strongly enough about the artemisia.... Take some very close-up photos and post them on GW's Name That Plant forum
http://forums2.gardenweb.com/forums/namegal/
and see if the experts there can give you a definite ID. Ask if it's a type that needs to be kept under stringent control. If it is, put it in the garbage.

If it's the bad kind and you decide to keep it anyway, whatever you do, don't let it go to seed! Round-up will kill most of what spreads by runners. But be prepared for it to take over the yard anyway.

What Duluth said too, and also what Inkognito said. One haul of plants does not a garden make :-) You'll buy more, not always so cheap, some will make it, some won't. But these are mostly perennials and although you will enjoy them for both foliage interest (I love hostas) and flower, over the long run perennials mostly are just a green mass in the garden. Really making an impact requires that you pay attention to the non-plant elements.

You're lucky you have well defined beds relatively nicely shaped, and likely won't want to change those. That frame will go a long way to making your composition look good no matter how you arrange the plants. But pay attention to your woody structure - that is, your treescape; maybe cull some and/or plant some new trees or shrubs for structure. Also consider whether you want some hardscape - that can make a huge difference - patios, walkways, fences, or other decorative focal points in the beds. One issue is that your fence is so transparent that the borrowed landscape behind it will visually overwhelm what you do inside the yard if it is a bit on the subtle side. All of that has a far bigger impact than plants do.

Your mystery plant is likely a Euphorbia. Check whether there is a milky sap when you cut a leaf or stem. If so, be sure not to get the sap into your eyes. It is not poisonous, per se, but very abrasive and can cause permanent eye damage. I taught my kids early not to touch those.

Karin L