I have kept BGKs with Cichlids for a lot of years… I currently have a 3 year old and a 5 year old BGK, each of which has been raised with Cichlids…
The Cichlids seem to ‘not understand’ the BGK and simply ignore/avoid it. I’ve never once seen one of my Cichlids show aggression to my BGKs in any way, except if/when the Cichlids are breeding. One of my BGKs is freakishly addicted to eating Cichlid eggs & fry and will get himself in some pretty rough situations trying to eat them. He does fine fighting a young pair of Dempseys but gets beat up pretty good against adult pairs…
I’ve kept BGKs with: Oscars, Dempseys, Gold/Silver Saum, Trimac, W African Jewels, Convicts, Severums and I’m sure others I’m not remembering…
I think your 7” Knife will be absolutely fine with 2” Red Devils… As the Devils mature they might get too rough for him, but I wouldn’t expect them to freak out and kill the Knife out of no where…