People have kept undulated triggers by themselves in 55 gallon tanks. They need to be the only inhabitant not only because of their size, but because of their aggression. They will literally kill anything you try to put with them. Maybe not when they're small, but when they get big.
In my 55 gallon, I'm keeping a female spotted boxfish, (maxing out at 6"), a bristletooth tomini tang, and a leopard wrasse. Some people wouldn't put tangs in a 55, but they kole tangs and bristletooths are the smallest of the tangs, and I believe it can be done.
With a snowflake eel and a dwarf lion, you'll be closed to being fully stocked. Maybe a dwarf angel would work too. There are many beautiful ones available that can stand their ground.