It depends on the area hunted for me. I try to use decoys consistent with what I've seen there. Ie I won't use canvasback decoys on a one acre pond in the green river valley just like I don't normally put mallards in a diver spread. That being said I normally mix puddlers with puddlers and divers with divers. Ie, Widgeon mixed in with mallards or read heads with bluebills. Occasionally I will put a couple widgeon decoys in a diver spread because I see widgeon with divers in small numbers fairly often.
As far as too big, are you seeing smaller groups where you're hunting or bigger? Early in the season we were running five dozen floaters and a dozen field decoys but Friday I had just twelve floaters out because it's late season and I'm seeing smaller groups.
I guess what I'm getting at is try and mimic what you're seeing where you're hunting.