Except for teal, my understanding is that it is best to keep the species separated. Teal are social, and will land in almost any decoy spread (my experience). We shoot a large number of Ringnecks and Redheads, and don't have any in our spread, so maybe it doesn't matter. A local hunter told me that it didn't matter what decoys you have or how you mix 'em.
My neighbor and I have only been hunting for 2 years, and we have mostly mallards, then teal, Pintails, then black ducks. We've killed a large number of Redheads, Ringnecks and teal, then a couple of mallards, one pintail, and a shoveler. Our two greatest harvest birds have been Redheads and Ringnecks, and we don't have any of those in our spread.
So, you question remains unanswered! :toofunny: