Since it's a 55 gallon, I don't think a fire eel would work. I've seen it done, and I mean.... It's been done lol. but it's certainly not ideal as they grow big ( and they grow that big sort of quick )
Gymnothorax Polyuranodon on the other hand I think would be perfect. They can grow to 3-5 feet, however I have never heard of one growing that large in an aquarium (and a freshwater one at that, since when they mature they move to brackish and when they get really big, I believe they even move to full SW).
One FW moray eel would be a great fit. They are pricey though. Shoot me a PM if you are interested as I will be importing some soon.
Not quite sure I follow or agree with, your logic here.
Fire eels grow to around a meter long give or take and whilst one in a 55g would be able to turn around it’s not ideal. And they do eat smaller fish.
Gumnothorax Tile grows around 2ft give or take and do do well in fresh water until older so is probably the best fit for a 55 but again not ideal.
Gymnothorax polyuranodon grow to over five feet in length so whilst they will be ok in a 55 for a short while, expect them to need something considerably larger (200+). Also, don’t expect in good conditions and when settled, anything less than the same growth rate as a fire eel (so your 55will not last more than a few years).
Yes, you can keep them in freshwater their whole life but if you were thinking of trying to breed them then you would need saltwater as it’s in their instincts to move to brackish as you say when they mature.
Don’t keep them with anything they can fit in their mouths as those crap eyes are made up for and more in the dark and a tank of guppies and mollies won’t last long.