I would try an engineer goby. They look like an eel and only get about 1 foot and are docile. I loved mine and have him in a 40gal breeder until my heater malfunction which killed everything
I kept a bright yellow 6" Cusk eel in my tank for 18 years then killed it by accident.
It would live in a small tank. The only problem is I only saw it a few times as they constantly hide.