I have 4 sengals with an oscar in my 60 gal. They got along well, the only problem is the oscar is pig when it comes to feeding. Usually I just throw in some sinking pellets at night once the lights are out.
I had my poly's in an 80 gallon with a mellow adult male midas, pikes, cons and red tiger motas.
Right now I'm keeping them in a 60 with one Gnathochromis permaxillaris, one Greenwoodochromis christyi and one Neolamprologus marunguensis. And more of each and the cichlids would be too territial for the polys.
i've had my bichirs with severums, a chocolate cichlid, psittacus, and a few eartheaters. they're never minded the bichirs even when my severums are breeding