my lfs has a display tank with a blackbelt cichlid, red hook silver dollars, (with big teeth), frontosas, julies, tinfoil barbs, big clown loaches, big synodontis and some other type of cichlid. this tank is like 10x2.5x3 (estimate). another fish store has a blackbelt cichlid with a black shark, big bala sharks, rainbow sharks, very small clown loaches, three spot gouramis a moonlight gourami and a black ghost knife. that tank is an 6x2.5x2.5. they also have another blackbelt cichlid with a heap of other south and central american cichlids including over 10 uaru, green terror, jaguar cichlid, convict cichlid and some others i can't remember. that tank is a 4x2x2 and is a growout tank for another cichlid tank that is 8x2.5x2.5, that has a mix of african and american cichlids.
oh yeah if your getting a 300 gallon with good filtration you could have more fish. 300 gallon is ]V[onster
a giant gourami might not handle a blackbelt cichlids agression, they are gentle giants that eat banana