In my own limited experience, firemouth aggression is more intraspecific (towards it's own/similar species) than interspecific (other species). This looks like a dominant fish beating the tar out of subordinate fish.
That being said, they can be aggressive, but not nearly as much as other South/Central American cichlids. You may want to try the smallest geophagus, blue acara etc.
You may want other medium central american cichlids, but as for someone saying put a "such and such" in it's a guess at best, depending upon a bunch of factors (size sex, "personality", water temp etc.). When I was a teenager, I had firemouths with fast moving fish like tiger barbs and found they had didn't bug eachother at all. I know that diverges from a pure cichlid theme. With stocking cichlids, other than giving them a lot of room, I have heard of many strange combinations working.
Just my ramblings and probably putting you back where you started.