hi, i just read this and i know its a few weeks old.i just wanted to say that i have a 12" long fin oscar and a 9" chocolate cichlid in a 55 gallon tank and they along just fine.what works for me may not work for you because all cichlids have they're own individual personality. i know almost everyone says" nooo a 55 is to small even for just an oscar".i disagree with that statement because i have had oscars alot of years,almost 30 yrs to be more exact. i think as long as you do regular water changes , keep the gravel vaccumed and have good filtration a 55 is just fine for 2 oscars or 1 oscar and 1 similar size and temperament tankmate. chocolate cichlids are much like oscars in size and temperament and make great tankmates for oscars.however it is better to keep only 1 oscar and 1 chocolate together. two oscars together may or may not work and the same is true with chocolates.chocolates get along very with other fairly peaceful tankmates except they're own kind.if you put two chocolates togeher there will almost always eventually be some serious fighting.they are slow moving most time but don't let that fool you they are a powerful fish and can get very agressive.an oscar is like this too.
i would never put even just one pair of oscars and 1 pair of chocolates together in the same tank unless the tank was at least 300 gallons.DO NOT PUT TWO PAIRS OF EACH TOGETHER especially in a 55.they grow to fast for that many to be together.i'm just giving my oppinion and what i've learned from experience.