If you get 2 hystrix or motoro rays then they wouldn't be too expensive. I've seen decent sized motoros for less than $100. I would do 2 rays, an arowana or some other type of larger centerpiece fish, and a mix of lesser aggressive sa/ca cichlids (just not your doviis, umbees, red terrors, etc). Or you could get a cichlid centerpiece and a bunch of smaller, less aggressive cichlids and two rays.