The snowflake eel and the lionfish should get along just fine. In my aggressive tank, this is the only fish that my eel is completely indifferent to. The other fish have had to learn to stay away from the eel during feedings. As for the crabs, they can always end up on the menu with large carnivorous fish. Some might say your tank is too small, but I definately think you will be fine, just means you will need to be more vigilant on keep the water quality good.