There are many variables that factor into the aggression level of cichlids in aquaria. Despite what you might read on various forums, don’t assume that just because someone says a particular species is really nasty that such a description applies to every cichlid of that species. It’s just simply not true. Are particular species more genetically predisposed to aggression? Probably.
If you’ve kept various cichlids (multiple species, multiple ratios, etc.) for a long time, you’ll know that cichlid behavior can’t be painted with a broad brush. Cichlids have unique personalities. I have kept some species together without any problem even when the community consensus advises against doing so. Does this mean you should abandon advice that you receive and do whatever you want? No. It means that if you choose to go against the consensus, be prepared to act when one of your fish is getting bullied (i.e., have a way to separate tank occupants either within the same tank or using separate tanks). Sometimes even this strategy won’t work, as some cichlids do their dirty work at night, and you won’t see it coming before it’s all over.
Regardless, if you’re new to keeping cichlids, you’re better off sticking with the consensus until you have experience. Better yet, if you’re new to cichlids, maybe begin with a single cichlid or maybe two or three, all of a different species. Avoid the normally recognized aggressive species until you’ve learned more about cichlid compatibility or have spent considerable time raising these fish. There are some species that are better than others to begin your cichlid keeping journey.