i had some periwinkles, prawns, and some anoemones in my reef tank. theyre all fine. well where till my hermits killed the snails for thier shells and the anemones got the prawns. i ID'd all my creatures first to make sure they could handle the tropical temps first. i also QT'd them first.
i live in the uk. the waters not rediculously cold, but its not exactly tropical. about 16 degrees C in summer. whatever you do, dont release anything back into the wild. and check your ok to take the animals from the beach in the first place. there are a lot of laws and regulations regarding removal of animals from thier enviroments in the US.