I’m assuming were talking about Crappie fishing here?
Unless braided line is being used for the main line, I’m not aware of a leader being necessary for Crappies.
I’m of the opinion most use 6 to 8 pound mono line as a main line, and some use lighter, then tie the hook or lure directly to that. If you buy some of the pre made trolling rigs, like the BnM Capps and Coleman, or the Mr Crappie brand, there’s nothing special about the line they use either. It’s just plain clear mono which seems to vary in size some with larger weights, but not by much.
As for the toothy fish wether in fresh or salt water, wire leader would be the best to use.
Some of that is soft and flexible enough to tie knots with.