The feed that I get now, a game poultry conditioner, has grit in it. It is usually what remains in the bottom of the feed pan. So I just dump it on the ground. I have a seperate bowl with crushed egg shells and oyster shells in it. All the birds prefer the egg shell to the oyster shells but I leave them out regardless. I've actually never offered grit, I still wouldn't if it wasn't in the feed but my birds are out every day foraging around and we have lots of small gravel and sand etc they can find.
With chickens, I wouldn't be concerned about mixing grit and oyster shell. I wouldn't for the waterfowl though only because they scoop instead of pick so may get more of either than they need.