I've had quite a few "eels". I currently still have a tire track eel. Tire tracks are VERY AGGRESSIVE towards other eels, and will kill them. I had to sell an 18" fire eel because my 16" tire track wouldn't allow it out of the substrate without it getting pummeled and bitten. They are also VERY aggressive towards fish that could be considered food, my tire track readily pounds 4, 5, 6 goldfish at a time with no delay. It's in a tank with 2 festaes, a dovii, and managuensis, 2 buttikoferi, 2 red pacu, a big clown knife, and an ornate bichir. Most other eels would certainly become food in that tank. Not a tire track.
Fire eels are much more practical if u have the size and want something that is a little more playful and fun (although my current tire track is SUPER active and all over the tank). They won't eat all your small fish and they're typically not aggressive towards each other so u can have multiples. BUT, like it's been stated they are the largest one you're gonna look at.
Peacock, zig zag, spiny are really where u wanna stay at. Fire Eels and tire tracks are much too large for a 55g if u intend on keeping it to full size. My fire's and tire tracks have breached a foot within the first year from purchase. The tire track I currently have is less than a year old and well over a foot long.
Hope that helps if you're still looking.
PS, here's a shot of the old tire track hangin with some bad boys of the predator world and a shot from when I got my first fire eel.