then you'll have to vacuum the gravel regularly, and you won't be able to purchase any sand dwellers, since you'll have no food for them. that gravel is meant for fresh water applications.
sure you can add rock a little at a time, but i wouldn't trust the lfs
that it is truly cured. (awful expensive for rock too!) generally what i see stores do is have a bin for their rock. they sell it off a little at a time and since it's sat in this tank for a month or more, they call it cured, however, they are regularly adding new shipments of rock to these holding tanks that's uncured. unless they have separate, heated systems for cured rock and new rock, and they have some procedure as to not mix the two ...i would be cautious and expect all rock to be uncured.
is this a regular thing, pulling rock from that tank with the fish in it?