how big is ur tank? i know nothing about fire eels other than that they get rather large(36 inches?)....when i was in your shoes and wanted something to put with my mbuna's i ended up adding catfish and a red tail shark
Try a search for the experiences of actual owners, probably in the General Aquaria forum since the eel is not a cichlid. From my reading of posts since 2005, the eels are predators and eventually eat the fish.
A 44G pentagon may not have enough swimming room for the mbuna and additions, what are the dimensions of the tank and how is it stocked?
Synodontis lucipinnis might be something to consider.