There is no certain answer to this question its often dependant on the temprament of the individuak fish.
I have a 100gal tank with a Convict(6inch), a Golden Severum(5 inch), a Salvini Cichlid(3.5 inch), a Pleco(7 inch) and a Rope Fish (7 inch). The convict is by far the most aggresive and he does tend to harass the Severum, he tries it on with the Salvini who although smaller is a vicious little beast who can stand off face to face with him if she wants. The Pleco and the Rope Fish are largely ignored because they are big and well armoured.