Gars, Bichirs, large Characins, large peaceful Cichlids would make a good tank mates. Each fish has it's own personality, but CKs are usually pretty mellow. However, there are some aggressive ones out there.
Not all Cichlids are aggressive. I'm not a Cichlid person per-say, but I've heard that Chocolate Cichlids aren't aggressive. And even semi-aggressive may work. Oscars are always hit or miss with aggression.