It is fine giving your hens their grit/oyster shells in either a separate container, or mixed in with their feed. I personally give mine a free choise, with pots of grit in various places of the run, but it is up to you personal preference.
Oh yeah I mixed my grower and layer feed 50/50 for two weeks. Now straight layers.
I have about 10#s of growers feed left so l mixed it 50/50 with scratch grains and toss it on the ground twice a day. They gobble it up. When that is gone I am thinking about doing the same with the layers feed.
That's how I introduced them to the pellets. GC