There are a fair few fish that are safe with Seahorse's and certain corals..the only thing you have to be wary of is fast, aggressive fish and corals that have stinging tentacles.
Fish eg's: Gobies (Clown Gobies etc), Firefish, Mandarin fish, Scooter Blennies, Hawkfish, Pipefish, Cardinal's etc
Coral eg's: Sun Coral's, Xenia, Gorgonian, Toadstool, Zoanthids, Mushroom's, Kenya Tree's, ***** Coral's etc
