Hi Jake,
The back of a peeler crab will be soft if it has already shed it shell, if not you will notice that it will be lifting off.
The way I put them on the hook, is to take the legs off ( I keep these for tipping aswell ) The legs will just pull of easily, the shell should either be off or if it has one it will pull of easily aswell.
A lot of people cut out the lungs which are on both sides of the crab just hiding in a small pocket above the legs, its up to you if you remove them though.
I cut them in half and whip a lot of baiting elastic around it before threading it on to the hook in a similar way to putting a worm on.
If using whole crab again remove the shell and whip it up with a lot of elastic and thread the hook though.
Big hooks are a must though as crabs are quite a large bait and could easily mast the hook point.
As for the species you can catch, I think the most obvious one would be cod in the winter, they go crazy for peeler crab but bass also love them. Flatties will go for it aswell and thats what I use the legs for, you can also catch thornies on them, smooth hounds, doggies and conger..
I hope that helps,
Sorry its such a long reply.
Tight lines
Mike N