I wouldn't try it, a 30 gallon isn't really big enough for two pairs of possibly spawning cichlids.
GBR's are mean little guys when they get near to spawning.
I would personally only go with one of those species and make it a trio of 1male/2 females so the females don't feel so overwhelmed. In reality, cockatoos are harem breeders and should have more females than males. I would also say, don't do rummynoses, they are fairly difficult and are very active and probably shouldn't be put in anything less than 3 feet. If you got a 30 long then you could deffinatly get a school of them.