For the longest time i had 2 african kalingo giraffe cichlids, and a yellow lab with my long-finned oscar, and never once had a problem with anyone getting bullied. They were all together since they were small. The kalingos are still with my long finned oscar, and im about to move all of them to a 125 with my other red oscar. The oscars are just about 9 inches, and the kalingos are right about 7 inches.
Everyone has always told me its a horrible idea to add africans with my oscars, but ive never once had a problem with any of them. They never fight over food, or territories. They all eat like pigs, and are happy, healthy, and colorful, and to me thats what matters.
But dont take my word on any of this, im still semi-amateur with fish keeping, and am just sharing my experiences with them. Every fish is different, and i might have just gotten lucky with mine.