generally echidna sp. are compatible with one another. the only consideration is the size of the tank and what/how many tankmates you have. since you have 55 and from what i remember you have lion/puffer/other fishes. i think it's going to put a lot of strain on the filtration system. there's also possiblity of aggression between eels due to a limited territory to claim. are you still planning to get 180g tank? ghost eel is more challenging to keep because they often refuse to feed.