in my experience with the cichlids i have kept (so far just angels, severums, honduran red points) their aggression - especially when small - is mostly towards each other. angels to angels, sevs to sevs, etc. conspecific agression. it seems like until they can win the battles over another of their same kind, they won't try to dominate other types of fish. when they get bigger and start to feel like the 'bosses' of the tank, then they will branch out. i had oscars too, but mine liked each other and everyone else, actually, so not much info from them.
i keep a 10" black ghost with a couple bichirs and 4 severums. nobody wants to mess with him now, but i think if the situation was reversed with large confident sevs and a small knife, he would have been beaten up at feeding time. as it is now with 4-5" severums, he just swoops in to eat like Batman and the other fish look on in awe until he finishes eating and goes back to his cave.