What size is the tank? exact dimesnions would help alot. If you're worried about the feed bill I would consider a Tire track, Armatus ( or unpatterned belly) Get just as big as Fire eels and tend to be alot less finicky in their feeding requirements. They are more liekly to take just abnout any foods. Fires tend to be almost strict invertibrate eaters ( worms,krill,shrimp, ect)
For companions I keep a fairly wide variety with my spineys. I keep Gar and a royal pleco with my fires, Bichir and pike cichlids with my tire track, a fantail goldfish and platies with my young leopard eels.
I would as a rule discourage cichlids, my tire track s about 12"+ and my pike cichlids are just large enough atm to be to big for him to eat. He's also very aggressive. more docile species of spineys do well with fish that are going to ignore them. I also avoid overly active bottom feeders like clown loaches and corys, but some people have no issues mixing these. I've seen fully grown fire eels in community fish type set-ups.
whatever you decide to do, my best advise is to get the eel first, and let it settle in a few weeks and eating solidly before adding more tank-mates. Getting most spineys to eat initially can be the hardest part in keeping these fish.