Hi Mr. Kessler,
I attached a photo below on how to hook a fiddler crab. The hook size I use most of the time with fiddler crabs is a size #2. If the fiddler crab has a long enough claw to grab your leader, you have to break half the claw off. Most of the time the fiddler crabs claws are too short to get to your leader.
As far as the shrimp goes, the pinfish and other baitfish can easily pull the shrimp off the hook. But the fresher and the better condition your live shrimp is in, the much harder to have it pulled off or even fly off on your cast. By keeping your bait well cooler, aerated and oxygenated better, your shrimp will be harder and crisper, and therefore stay on the hook better.
The best way to hook a shrimp is having the hook going through the first digit of the tail. When hooking the shrimp in the head area around the horn, the shrimp can easily be pulled from its shell.
Thank you for sending in your question to The Online Fisherman.
Captain David M. Rieumont