you just need to put in some good decorations and hiding spots and make sure that no new fish you add are small enough to be eaten by any other fish in the tank, especially since you already know he tries to eat smaller fish and wont leave them alone. in a tank that large as long as you have enough dithers or tank mates for them to spread their aggression out on and lots of hiding places for them to claim their own territories then you should be fine, and some chasing and nipping is normal in any cichlid tank, especially a newly set-up one where they are still figuring out their pecking order, just watch closely and separate if one of them starts to show real damage, and those barbs might be ok for now but once that O reaches 6-8in then they will probably be lunch if he can catch them (though i have had Os that size living with tiger barbs they were raised with in the past i was lucky and most situations like that dont work)