The only Knife fish I have much experience with are Black Ghost Knives… but I have raised several of them in tanks with SA/CA Cichlids and kept them together for a number of years…
I’ve raised BGK’s from 4” or so side by side with small Jack Dempseys, Oscars and similar Cichlids… My consistent experience has been the Cichlids ignore the BGK and when the BGK comes around the Cichlids act as if they do not understand what the BGK is and wait for it to leave… Mine have eaten side by side their entire lives…
I recently sold a 12” BGK and currently have a 12~14” BGK. The one I still have is over 5 years old, the one I sold was a year younger. Both of these were raised with SA/CA Cichlids and lived with SA/CA Cichlids the whole time I have had them.
I would like to add that twice I’ve tried to keep baby BGKs together and both times they immediately fought to the death (or separation). I’ve never mixed my large BGKs.
The worst sign of aggression I’ve seen between Cichlids & BGKs have been when a young pair (4~6”

of Dempseys spawned and a 10~12” BGK decided to eat the fry. The BGK dominated both parents while also grabbing fry until the entire batch was gone. I’ve seen this exact thing take place at least 3 times. Having witnessed this I now believe that the potential aggression/toughness of BGKs is highly under rated…
My 12~14” BGK currently lives with an adult pair of (Blue) Dempseys, a small group of Dempsey grow outs, a young adult pair of Elliot’s Cichlids (very similar to Firemouthes) and a large spawning group of young Geo Orange Heads. I’ve had at least 15 spawns amongst the Geos (have seen mothers holding) and have had eggs from the Elliot’s Cichlids but have yet to see a single fry in the tank, which I suspect is because of the BGK.
Based on the personal experience of raising a number of BGKs with Cichlids and keeping them together for a number of years… I think that BGKs and SA/CA Cichlids make great tank mates… that being said some common sense much be applied to balance sizes, provide an adequate environment and avoid excessively aggressive tank mates