I have a zig-zag eel and a peacock eel. They are 2 different types of eels and they live in the same tank together just fine, in fact they often times share the same hiding place. However, I must warn you they are really too small to be put in with those bigger fish. At max size they will reach between 7 to 12 inches. I keep them with ghost shrimp and while they sometimes chase the shrimp around, their mouths are way too small to eat them. They mostly eat black worms. The peacock eel burries himself everyday but it is usually around the same spot so I wouldn't count on the eel to stir up the sand. The zig-zag eel doesn't bury himself at all, he prefers to hide next to the driftwood or in a fake log I bought him. They really are not what I would call aggressing at all, but if your fish are less then an inch or so, when the eels are full grown they may pose a threat.
If you wanted an eel that wouldnt be a possiblity of getting eaten then I would go with a tire track eel or a fire eel. But both of these can reach over 3ft and need a bigger tank then a 55 gallon tank. They need something more along the lines of a 75-100 gallon tank. Plus you would have to wait until they were bigger to add them to the tank unless you wanted them to be expensive food. They are a really fun and interactive addition to an aquarium but I am not sure if they are the best choice for a tank will bigger fish while they are still small.