The tank already has a high bioload, best to remove the parrot. The rainbowfish there is a schooling fish you might add 6 more of them. Don't even think of adding oscars. Keep the others undisturbed. Is the discus really okay? For bottom dwelling species, you might try to add 4-5 bengal loaches (sold as queen loach in Galiff)? Also, with a high bioload, you have to do frequent water changes. Nice severums, BTW.