ive seen them kept together in a large(180 gallon) tank and the fire eel was huge. approaching 3 feet and he used to torment every fish in the tank at feeding time including the jd and a few green terrors. as long as the tank is of sufficient size and the eel is large enough not to be harassed or swallowed it should be just fine.
The fire eel is gonna definately dwarf the JD...lol....i'm not sure about the growth rates of them though. I would be more worried about the Fire eel eating the other fish...although their mouths do look relatively small compared to their size....:S
I currently have a foot long tire track eel with a fairly large midas. Tire tracks and fire eels are nearly the same. They seem to hang around each other without any grief. The midas is as mean as any jack, but never bothers the eel.