Was fishing off of a pier yesterday with light tackle and fresh shrimp, trying to catch croakers and whatever else will bite.

Three rods were rigged with line, snap swivel, circle hook (snapped on). In that order.

I had a fourth rod that was rigged with line, sliding egg weight, metal leader, circle hook (snapped on). The rod with the metal leader rarely ever got struck.

I know metal leaders will probably scare fish, so when should I use them?

I ask because i have always heard that saltwater fish will bite off line, and coincidentally, I lost some terminal tackle when an unknown fish bit off one of the rods that did not have the metal leader.

Question 2: is it a bad idea to put the hook directly on the snap swivel/metal leader?

I'm not gonna be your best source on this, but I will let you know my opinion.
In salt water I only use metal leaders because you never know what is going to take your bait. I figure better safe than sorry. I make most of my leaders so I can not answer your question about the snap. However with my smaller rigs I will put a small circle hook directly to the snap. I usually am just trying to catch my bait with this rig and have had no problems with it.
This is just my 2 cents. I am a freshwater guy, so don't put too much Sri k in what I have said, just my personal experience

If you're targeting toothy critters, use some wire or heavy monofilament. 200 to 400 lb. test mono works pretty well. And lose those snaps; they are only good for failing at the wrong moment.

Those croakers, whiting, reds, specks, drum, catfish, and most other inshore species don't require wire. Just use a 18" or 2' length of 17 - 20 # fluorocarbon and tie your hook on directly.

I stick to a 20lb flouro leader most of the time but always keep a few spoons and jig heads in the tackle box rigged with wire in case I get into Spanish Mackerel in the surf.

X2!!!!