More likely to see a AKF 6-8 than 8-12 (never heard or seen of one over 8inches actually)
I have kept one with cichlids int he past, but more peaceful cichlids like severums, blood parrot, angel fish, firemouth, and a krib (yes weird ass tank, long time ago). Everyone leaved the knifefish alone and he was ALWAYS out and about and would eat anything, including flakes on the surface.
I have one now with polys and ctenopoma and the largest ctenopoma chases the knifefish occasionally. This one is a lot more shy that the one I had in the past, but it still comes out during the day if I sit still for 30 seconds. It was a pickier eating, only eating "real" food (aka not pellets or flakes) but it has recently come around to eating pellets and flakes.