Ben:
Welcome to the forum. I have a groundwater pond dug in sandy soil and I can relate to the ups and downs of the water level. For fish, you are going the correct route in stocking the feeder minnows first. One thing, research where you are getting your minnows from. You could introduce unwanted species along wtih the minnows pretty easily.
I'll defer to some of the experts here on what to stock fish/minnow wise, and where to get them. At this late in the season, I think the only minnows that will pull off a successful spawn or 3 this year will be Fathead Minnows (FH) that I think are sold in your area as crappie minnows.
As for structure, yes you should have some. The minnows and baby fish will need something to hide in, and some structure will be needed for the minnows to spawn on. Fish will orientate to the structure as well, and it will make finding the fish in the winter easier if you are planning on doing any ice fishing.
The rye grass will do a good job of covering the soil, but if you are having a hard time getting it to grow well, you should have your soil tested and apply the recommended fertilizer and adjust the pH if necessary. There are places that sell seeds for shore cover, but the prices might suprise you. A couple are Spencer Nursery and JF New. I don't know of any good fish suppliers in Wisconsin, but I'm sure someone here does. Buying the minnows from a supplier will be cheaper and you'll most likely get better quality fish than buying them from a local bait shop.
Here are a couple of things for you to read and look at until the experts start responding to your questions.
Structure Aeration Fatheads Acronyms Structure Placement Happy reading!
