i own lots of cichlids like:
astronotus ocellatus, parachromis managuensis, parachromis motaguensis, vieja synspilum, cichlasoma umbriferum, cichlasoma festae, Aequidens rivulatus, Red Parrot, geophagus tapajos and datnoides
my oscars were only aggressive when breeding. rest of the time the were very peaceful
jags could get mean against certain fish. they killed 2 red parrots and my female gt. they also beat the **** out of my male gt which was a few inches bigger. i had to give him away. but on the other hand they got beat up by a pair of motaguensis all the time, so i had to split both pairs.
umbriferum was not as aggressive as i expected him to be ...
compared to his size, my male festae is the most aggressive fish in my main tank. he killed 3 other festae of the same size and ripped out their eyes lol
also the motaguensis were very aggressive, but i had them only for a few months