I'd recommend either shovelnose strugeon, or sterlet. All the others get too big, or are endangered, or like to go out into the ocean for long periods of time. But you probably won't ever see a shovelnose sturgeon anywhere, so sterlet. They reach about 3'.
Sterlets can tolerate up to 75F, but lower is better. I keep mine at 60F. Getting them onto Sterlet sticks is a pain. I've been trying to wean mine off of black worms, but they're stubborn. Unfortunately, they seem to become emaciated much quicker than other fish, so it's even harder to get them onto prepared foods.
It's not a good idea to have tankmates, unless you can separate during feeding time, otherwise they won't get a fair share. Most fish will pounce on food, but sterlets will mostly swim around the bottom until their barbels hit food, and then eat it.