FOR ONE IT's a 200g TANK.
As for the fish killing eachother, not necessarly, I've kept my FireMouths with a Green Terror and an Oscar in the past. This set he's talking about doesnt exactly mean every fish will kill the other. It fully depends on each fish's temperment. I have two 6" Hybred Texas Cichlids in with my FireMouths, and my FireMouths hold their own in the tank.
I doubt I would go with an oscar in the tank, I'm just not a fan of them anymore. I would go with only a pair of Jack Dempseys, 4 FireMouths, single Green Terror, and probably 2-3 Pictus Catfish.