They are all sturgeons Divemaster. The Beluga and Kaluga are Called Huso huso and Huso dauricus, not acipenser but they are classified as sturgeon. So are the Paddlefish and Chinese Paddlefish.
About Paddlefish not surviving over 5 years, I have never heard of that. A local zoo here has huge ones, I don't know how long they have been there. An acquaintance of mine has a couple and I have one right now as well. I was worried that my Beluga could eat it but it leaves it alone. My only worry is it being eaten by a heron, it is approximately 50cm long now, it's probably out of the danger zone but I've lost a 40cm Stellatus before. I'm one of those people Chub_by knows who owns an Atlantic sturgeon, so I can confirm that they are legal to own and available (though rare). I would assume they would be available in the US or Canada, since that's where they come from.