If you spring for one of the more peaceful, easy to keep morays, such as the zebra or snowflake, most of your fish, often even ones smaller than the eel's mouth, will be just fine, but don't try too many inverts. One of my eels had numerous bad experiences with those. He was stung by an anemone, then in his pain he knocked over some decorations, then after the anemone was gone he got brave and started chewing on crab shells as if he were a dog with a bone, I've got to say it was cute, but I'm sure my hermits were not quite as pleased. Plus, and I have no proof of this so I can't truly blame him, a while after his aquistition my scarlet cleaner shrimp disappeared, but I don't think it was him because the shrimp used to clean him, but you never know...