the lion shouldn't touch the snails and hermits. emerald crabs... possibly, yeah, but let them do their job of getting you through your first algae bloom, mow down some of the bubble algae and then think about putting in the lion. Be careful what type of trigger you end up with because some of them are known to nip at the rays of lions and some of them will crush shells of inverts. Cleaner shrimp would not be a good idea for you because they make excellent snacks for those fish.
To give a suggestion as to what triggers you might think about looking at: Pinktail, Niger, or Bluw Jaw. They're all fairly harmless to most other fish and are all nice looking additions to a tank.