Great thank you so muchYou can do what you want & as long as the shells are cleaned properly & don't have any sharp edges, they would provide some natural looking cover for smaller T's.
My only concern would be, not putting too many in & not allowing the T to climb over hard edges. That is have the shells edge down, smoth side up. So your T does not harm it's self by constantly rubbing against hard edges.
Edited to add. The right sea shell could provide a nice looking water bowl.