I have had my current flame hawk since 1999 and had another before that for about a year. Small hermit crabs always experience a slow attrition in my tank. I think he gets them when they are changing shells, and that is why they can last for many months. My FH does not bother my snails. My serpent star sometimes eats snails. I have had hermit crabs kill them too.
I have had a coral banded shrimp in with my FH since 2006 with no issues. The CBS was bigger than the hawk when I got it (by design).
My first FH used to follow my hand around the tank if I was moving rocks so it could swoop down on any amphipods I disturbed.
Every fish is different. If yours likes crabs or snails, there is not much you can do to change that. Keeping a fish well fed with food you want it to eat does help to keep it from eating things you don't want it to eat. That holds true for others, like keeping herbivores from picking on corals.