Ducks on one side, geese on the other with that amount of decoys. Also, ducks will fly over geese but geese aren't overly found of flying over puddlers. Have seen a few mid-air collisions over the years. I'd try and set the mallards so that your favorite shot, quartering right to left or whatever, is there if you're target is the mallards.
First it rained, then it blew, then it frizz, then it snew --- from an oldtimers gunning boat ~~ He understood what it's about!