hmm Im leaning towards severums. But from what I read some are aggresive punks and some are not. I never had a problem with my sen and my Oscar before but once again their personalities vary too much from fish to fish.
I keep my 8" ornate in a planted tank with angels, gourami, a severum and a green aro. I had 2 severum but the larger one bullied everybody so I moved him to another tank.
Mine is with 2 florida gars, a dumb blue acara which will not die, pleco(I know), and a raphael(which is totally awesome even though he never comes out).
I have had success with dack dempseys, severums, climbing perch, african knife fish, syno cats, gars, big clown loaches, even kissing gs, a pleco that has worked for me is L333. I have had mixed results with the bristlenose sp, some go after the slime coat it seems. But I keep mainly upperjaw sp. A hungry endli is very capable of eat some of what I listed. Festae is a no go, tried it worked for a while but she took over.
I've seen it done before, 3 huge ornates and 3 massive senegals in a 3000L tank with [among other things] a group of large adult discus. Starting out with larger discus would be a good idea though.
I keep my big ornate with an asian aro, dats, eartheaters, severums, uarus, a royal plec and a tyre track eel. More peacefull south american cichlids make great tankmates for polys.
bichers and pretty much all other fish can live fine in brackish water, the salt is actually good for them... I have mine with gars and a mild tempered dempsey in one tank and my other with a smaller marble gudgeon and a couple columbian shark... all seem to do well as long as size is appropriate with their tank mates
i got silver dollars, fire eel, giant gourami, silver arowana and a common pleco together with my bichirs. (2 delhezi 2 ornate 1 senegal 1 albino senegal)