A ton of filtration isn't the answer to overstock, though. Adding another big cichlid is going to dump such a bio load in there, never mind the aggression factors mentioned above. You have caves and hiding places, which is good, but should your fish who were there first be forced into hiding? Like the others have said, that Midas is going to wreck everything. Oscars get too big, and besides, they're such tank wreckers that they will have all your caves and stuff turned upside down in a minute when they get large. I can't begin to tell you how many heaters and intake tubes I've lost to wayward Oscars.
I would make a different choice.