anything over 3" that is docile.. aviod any cichlid potential pairs.. and if you go with any cichlids at signs of aggression re-home it, they will get worse once they realize the eel won't return the aggression. there are exceptions to this rule, but not many.
If your going for a planted tank type set-up I suggest alot of driftwood for the eel to hide in. and go with larger tetras/rainbowfish. imo Denesoni barbs would still be a deffinate possability. You could also try some very docile cichlids like rams ( I would go with bolivinas for a few reasons, larger then germans, less delicate. and just about as colorful) Otherwise if you like discus they might be an option.
fire eels are alot of hit and miss with tank-mates. I would be worried most about the eels getting attacked then them attacking other fish. I would also suggest getting tank-mates that won't outgrow a 135. the eels needs will require a larger tank. But even 40" fire eels rarely take fish that are 3" or more unless trained to eat fish. So when looking at an upgrade your worried what size the eel will need, not so much it's tank-mates.
avoid other bottom dwellers for the most part as well. Plecos can be downright viciouse, particularly commons. Royals or other Panques would likley be okay tank-mates. even though they have fairly good sized teeth they are generally 100% herbivorous, because most plecos are omnivores and some carnivores.
as for what would I put with my eels? exactly what i have tbh.
Florida gar ( alligators are way to aggressive, and the others still get very large)
Royal pleco
Pike cichlids ( venezualans, though i'm not sure if I would suggest it yet since we haven't hit the 6month+ point yet. But so far so good. And these guys are one of the more docile pike species from my research)
the gar and pikes move all over the tank, and the eels will hide until they see me, then are in the corner begging with the pikes for food. atm the pikes share the pvc tubes with my eels. and the gar tend to nap ontop of the tubeing.