Fire Eels typically do well with anything that doest try to attack them, they don't take well to attacks from being scaleless fish.
I'd say as long as your other fish are smaller and/or not aggressive then you'll have no issues.
You'll need a pretty hefty tank for one though.
I'm looking at an 8x2x2 to house Fire Eels right now, ferocious appetities too, thought they wont touch anything that isn't bloodworm for me.