Good.
Never trust what a feed store employee tells you. They are NOT EXPERTS>
Read the guaranteed analysis tag. If it is over 2% calcium, it isn't appropriate for mixed flocks.
It is unusual for an All Flock or grower feed to be 16% protein (though I have seen it in organic feeds). Are you sure the 16% isn't layer feed (4% calcium)?
The only All Flock feeds I've seen from various mills was either 18% or 20% crude protein.
If, for some reason you want to temporarily...
Any birds not actively building egg shells should be getting about 1% calcium. Layer is 4%. Take the rooster off of the layer now.
A grower or All Flock crumbles with oyster shell in a separate container is how most people handle your situation.
However, protein can be an issue.
Layers need...