Your problem be is that you were looking to have a good number of fish with some eel and with having a 55 gal tank to do this, isn't being realistic. For yes, your a number of eel that can be managed in a 55, only other problem is looking for some fish selection to go with it.
Like one of the real ideal eels for this size a tank, is the Caribbean chestnut moray for it grows not more over 12" and is able to eat most foods. Maybe you want to look into having a pair of these.
Then there be what is called a Lipspot moray and you would not be able to keep fish in with it.
Abbott's moray eel is another>
Slendertail moray is another eel that you can manage a pair in a 55
Grey moray is another eel which could fit well in a 55
As the Ghost moray
Jewel moray
Whitespot moray
There are a number of others I could mention but for the most would be not easy to obtain and other then a dwarf eel, one comes to mind which would and could work better then any of the above that are listed and that is the Goldentail moray. And depending on the fish you want to go with this eel, must be a max size of 4-5". And depending on those fish, you might manage three in all to go in with this eel. Your top best be closed good and tight with no openings.
Again, your a small tank, butterflyfishes in general require large swimming spaces in to possibly doing better.