IMO, I'd have to say no on the Snowflake Eel, just because of their adult size. I know alot of people say it'd be great, but nothing less than a 90gal, for a Snowflake.
You could do Dwarf Puffers, such as the Valentini, in a 55gal but then you can't have any species of Lion.
Puffers are notorious for picking at the fins of Lionfish.
You're very limited to what you can do, aggressive wise, in a 55gal tank. IMO, if you're wanting to do an aggressive tank, a 55gal is pretty pointless.