I am by no means an eel expert, but I have had my snowflake for a year now, so I can help a little bit...
Will it eat hermits and snails? Nope, at least I have never heard of one eating those. I know mine has never even looked at one, so I doubt that any do.
Will it eat a cleaner shrimp? This is usually hit or miss, although most eels do know the difference between a cleaner shrimp, and a meal. Cleaners all have white antenae and I believe this is something that the eels clue on too, as mine has been living with two cleaner shrimp (neither being large) for about nine months now and the eel even lets the shrimp craw into its mouth to clean his teeth after a meal.
Fish a meal? I think the big thing is, try to find an eel at the LFS that is already eating frozen foods. If you can do this your fish should never be in danger. My eel opens his mouth when the fish come by, but never moves at them, so I assume this is a "warning" sign to the fish to get away. My eel does not bother any of my fish and never has so I guess this one is hit or miss as well, although on eating frozen has a much higher success rate.
Eel proofing your tank? Just make sure that there are no escapes for the eel to get into. You already have the egg crate, so thats a big plus. Make sure if you have overflows, that you make sure the eel cannot be sucked through as they are known to end up in sumps. All I would do is add a piece of egg crating around the overflow to keep everything a half an inch away from the edge or so. This is about it.
Good luck, and I love the SFE I have, its a great addition to my tank and is a model citizen. I think snowflakes are a great addition to most tanks, as long is there is nothing that is a known eel eater in the group (groupers are known for this). Have a good one!
