Sturgeon get big, yes, but I don't think ever that big. Also, sturgeon are bottom feeders, so they have more of a "sucker" type mouth at the bottom of the head. Not sure if it would ever be large enough for a person, but even if it were it would be a
very awkward operation to get a man inside one of these fish. Can't remember that story in detail, did it mention being consumed into the sea? Or did it mention some form of fresh water environment? That would make a big difference too when thinking of potential fish species.
Regarding the Smithsonian episode, perhaps a megalodon? They're big, lots of teeth, predatory. Didn't check the size reported with estimates for megalodon, but I know that they were big, twice as long as the largest great whites. That'd make them around 50 feet, which fits the story. Being a shark, one wouldn't have to surface for air. Sharks don't have scales, per say, and have relatively tough skin. I vote
Carcharodon megalodon !!!!
It couldn't be a whale shark as they don't have teeth and eat plankton, and the Smithsonian should be able to recognize one of them. Also, one should consider that though "When cut open an animal weighing 1,500 lbs. was found in is stomach." could also mean that it was carrying offspring. Great white sharks (related to megalodons) are viviparous, they give birth to 2-6 fully formed pups which can be up to 1.5m long. So...maybe there are still megalodons roaming the oceans...schweet...
Cheers!