I've had eels off and on over the years. While most eels only curl up and stay in one spot, Salt Life is suggesting that it's too much of a bioload to keep a 2 foot eel in a 46g tank. Which, I agree. If you keep up with water changes, and increase your filtration, you should be ok, however I don't suggest it.
Engineer gobies are really sweet, they get big, and they are more entertaining IMHO. However, if you really want an eel, I highly suggest just getting one small snowflake.
BTW, there is no such thing as a reef safe eel. They will eat hermit crabs and snails and other sorts of critters when they are hungry.