Niger Trigger - will outgrow the tank
Snowflake Eel - will outgrow the tank
Dwarf Lionfish - no problem
2 Ocellaris Clownfish - may be eaten by lionfish
bubble tip anemone for the clownfish - unlikely to do well in FOWLR - need reef lighting, very clean water
I think you need to decide what type or species of fish is most important to you, which will live long term in a 55. We can help with stocking around your most favorite fish.
No trigger is going to be good in a 55, they are too big, active and territorial. But some filefish and the dwarf (sharpnose) puffers are related to triggers, and show some of the same cool and interesting behaviors and have some of that trigger "look".
You might be able to keep an adult snowflake eel in a 55, but you really wouldn't have the room for other fish. And a larger tank would be much better. A suitable sized eel is the golden dwarf moray, but these are pretty pricey.
Dwarf lionfish are great fish, and one or two would do well in that tank, but you have to consider compatibility of tankmates. Anything small enough to fit in the mouth is likely to be eaten, and some fish pick at/on lions.
Ocellaris clowns are a nice choice, as is a bubble tip host anemone, but then you are going more toward reef conditions, and a different set of tankmates.
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