You won't be able to keep scallops, they're filter feeders that starve to death in all but the largest tanks.
Most urchins get a bit large.
Anemones will often eat gobies.
Many crabs are opportunistic and will kill small/slow/asleep fish.
For advice on what inverts could go in a small tidepool biotope, perhaps try Reefcentral.com? They'd know plenty about saltwater inverts and small aquarium-type fish.
I'll warn you, a 15g is a nano tank in saltwater, and small tanks are harder to maintain.