Don't worry about it at all.
When I was younger we never had anything but mallard decoys and we killed divers consistently over them.
Since we have diver decoys now, we mix them in with our puddle duck decoys. Around here that is how they all sit where we get the divers on the flood waters. Sure hasn't hurt us at all mixing puddle ducks and divers. Scouting and watching the birds in your area will normally tell you what they want. If you get a few copies of D.U. look in the back at the Greenwing section. They have a deal called Marsh Watch. It will normally have all sorts of species mixed together. Alot of times these are puddle ducks and divers.
Do what you want to do and if it pleases you to seperate them, then by all means go for it.