Fish General

Yes. Lamprey and hagfish are both jawless fishes. They are the only living members of the taxonomical class Agnatha (Greek for “no jaws”). 
No. Although sturgeon and catfish can be found in the same habitats and they both have barbels (whiskerlike growths extending from the jaw), they are not closely related. Sturgeon are much more primitive than catfish.
Sturgeon are the largest of the freshwater fish. The beluga sturgeon in Russia is the largest freshwater fish in the world. The white sturgeon is the largest freshwater fish in North America.