I'm not a cichlid guy personally, but I've had my little ABK for a while now. He's been kept with a variety of tankmates, all of which have treated him well. The only exception was my golden clown knife. When the gold knife was small, my brown knife used to bully him a little bit. When the gold knife outgrew the brown knife, the tables turned, and the gold knife was much rougher on the smaller knife.
In the past, my brown knife has been housed with convict cichlids, a medium silver arowana, and currently a small fire eel and young datnoid. His constant tank mate has been my sen. bichir. Like I said, the only thing that ever bothered my knife, was another knife.
I don't know anything about the temperments of black belts or salvini's, but if they are like every other cichlid, I imagine there's a decent amount of aggression? If it's standard cichlid aggression, the knife should be ok.
Best of luck with whatever you decide.
