Canary grit is fine but you may find it to be very expensive compared to feed store grit or a scoop of dirt out of the yard. As for the oyster shell, that is not typically added until they start laying, when they'll use the additional calcium to make eggshells (sort of, I'm sure it is more complicated than that!). However, if it is offered free choice there is no reason to think they'll take more than they'll need. The concern is if there is too much calcium in the feed (feeding layer feed too early) and they get it with the food.
We all do the best we can with what we have available, and somehow most of the chickens grow up just fine!