They seem like they can be aggressive, especially in a smallish aquarium. There is probably some variation from species to speces. I've kept 3 species before; blackbelts, synspilums, and reganis. I found the blackbelts and syspilums to be pretty aggressive, not as nasty as a midas or trimac, more along the lines of a Texas Cichlid. Quite capable of inflicting casualties in a smaller aquarium, but if given enough room tend to calm down somewhat.
Reganis tended to be quite a bit mellower in my experience. They were at the low end of the pecking order in a mixed cichlid communty I had. But it was a fairly aggressive mix.