My father has made probably 10's if not over a 100 different custom screening boxes. he made me a dozen or so smaller ones for me last summer, here's a few. Like you said every species is different and thats why there is a need to have lots of different size screens to accommodate these species. We also have this interesting hammer mill contraption that we found at a farm sale probably 10-15 years ago. That works great for putting milkweed pods in and busting them open. However, you can do just as well manually with a few nice Rubbermaid Tupperware bins and some old window screens.
As for the grasses and removing the fluff. If you are planting a lot and want to use a seeder sometimes it's just easy to buy professionally cleaned seed. Or just deal with the fluff. Those ones are the trickiest.