Fire eels are one of my favourite fish. I plan on keeping some in a big community tank one day with aros.
They are incredible escape artists though, so if you ever keep them make sure ALL holes are covered or else you will wake up one day to a dry eel on the floor!
I have only seldom kept them together but when I did I never had a problem. I would suggest they be the same size. The largest fire eel I raised was bought at 5 inches and died at 33-34 inches. He was expensive to feed (500 dew worms a month when he was over 24"). I find that the ones I have now grow a lot slower but I have them on baby shrimp (bought at an oriental grocery store) but the cost of raising them is a lot less than when feeding them dew worms. I have 2 at about 12-13" and one at 20 plus inches.