I've got some big boys together without any problems to speak of (except a case of ich). I will say that it definately is possible and has been the most rewarding experience I've had in this hobby. With that said, it is not for the average hobbyist and requires a lot of research, work, and effort to maximize your chances for success.
My philosophy centers around keeping the fishes "happy" to minimize what I call "nuisance" aggression. I consider nuisance aggression to be aggression towards tankmates. I feel you can't take away aggression in these fishes, only minimize it and let them release it in a healthy way. You keep them happy by providing them with a "house", excellent water, and a lot of food. I feel live foods from time to time allows them to still be a predator (again, keeping them "happy") and lets them release chase/hunt/murder aggression in a healthy way. There have not been any murders and no squaring off or lip locking. Only an occasional nudge when one of the Oscars gets in Tiny's way during feeding.
I keep the following in a 190 with a 50 gallon wet/dry sump:
1 Parachromis managuense (Jaguar) male about 12"
1 Parachromis managuense (Jaguar) male about 9"
1 Parachromis managuense (Jaguar) male (I think) about 3"
1 Parachromis friedrichsthalii (Freddy) male about 11"
1 Parachromis loiselli (Loiselli) female about 6 1/2"
1 Amphilophus festae (True Red Terror) female about 7"
1 Aequidens rivulatus (Green Saum or False Green Terror) male about 6"
1 Astronotus ocellatus (Tiger Oscar) about 7" to 8"
1 Astronotus ocellatus (Tiger Oscar) juvenile about 4 1/2"
1 Synodontis eupterus (Featherfin Catfish) about 6"
1 Synodontis longirostrus (No common name I know of) about 4"
Half of these fishes, more or less, have been together for at least six months with zero problems (the first 3 months they were in a 55 while I was getting the big tank ready and set up). The rest have been added along the way as I've found good specimens that were on my list. This is pretty much my dream tank and contains what I consider to be the most beautiful of the CA/SA cichla (save for the Red Tiger Mota female).
A more detailed story is on another forum. It explains my philosophies more in detail and contains some pictures. You can check it out if you like, but don't run anybody down over there, they're good people and very knowledgable (especially about African Cichlids). They just don't share the same philosophies about stocking that some here follow (specifically that my tank will eventually end up in a prison yard race riot or an orange mushroom cloud, lol.)
http://cichlid-forum.com/phpBB/viewtopic.php?t=157173&postdays=0&postorder=asc&&start=0 (the first few posts may not make sense as this thread was started by a mod moving some of my posts from a different thread)
I've broken a lot of rules in the "conventional wisdom" in keeping these kinds of fishes together (too many big aggro fishes, normal temps, males from the same species, live foods, etc.). According to conventional wisdom, all these things should contribute to more aggresiveness. But, I haven't found that to be the case. I don't advise this setup for everyone, mostly because most probably aren't willing to devote the time and attention it takes to be successful. I'm no expert, I consider myself an amatuer icthyologist, lol. Don't take this as advice, it is only my persoanl experiences. It may not be the same for others. It's a ton of work, but in my experience, well worth the efforts.
The only advice I could give is do a lot of research and talk to those who have done this kind of thing successfully long term.
And here's a pic for listening to my ramblings...
This is Tiny...
***Edited To Add*** I forgot to mention that the Loiselli and Friedrichsthalii have mated and laid eggs. The eggs disappeared after a few days, but the important point is that during this time, no murders, no fighting, no aggressiveness to speak of.