If you consider yourself a responsible fish keeper, test water on a regular basis, perform regular water changes, have a hidden credit card for your aquaria purchases, you can keep 2-3 in a 75 gallon tank by themselves or with a cleaner fish. One can be kept with other docile species. they do not require companionship.
If you keep paddlefish you will quickly observe that 1. they are way-cool fish and exude an ancient feeling about them. 2. They are really stupid and have no personality whatsoever.
Moontanman has too many fish in his tank for my liking but he has fewer life responsibilities than I do, more time to screw with his setup. I have a full time job AND grow out thousands of paddlefish every year in 200,000 gallons of water. My home setup is a small reef tank that I scrape the algae off the glass every fall to see if anything has survived. Damn flame hawk is going on 8 years or I would have taken that setup down years ago.
Understand that even the fish I cull as runts occasionally will sprout into big-ass fish. We will take them back if you have no where to go with them. This rarely happens. I have been doing this for seven years now and am pretty good at culling.
We are learning new things almost daily about these fish.