Gymnothorax can usually be kept with cleaner shrimp. I personally favour small members of this genus over Echidna for inclusion in the reef tank, as Echidna usually eat crustaceans. This is probably resulting from the fact I usually only keep "tough" fish that are unlikely to be eaten by a small Gymnothorax without a fight.
In any case, feed them dead foods unless you have no other option. I cannot stress this enough. They will be much less likely to attack living animals if they are fed dead foods and plenty of them.
However, perhaps the best larger muraenid (excluding the ribbon eels) for the reef tank is Gymnomuraena zebra. A gentle, hardy giant, this animal isn't suited for a forty gallon tank but can be kept in tanks of over a hundred gallons. Extra water volume to dilute waste perhaps takes first priority over living space, as they are inactive. Won't eat even the smallest of fish, but can be expected to eat true crabs and shrimp, possibly excluding cleaner shrimp.
A sometimes overlooked issue with inclusion of eels in a reef tank is that they can produce a great deal of bio-load. If one's water changes are kept up (GFO and refugia can also be invaluable) it shouldn't be a problem.