2000 gallons is not close to enough room for 1 sturgeon, let alone 2. You could get away with a Sterlet for a longer time- they only get 4 ft. There are Sturgeon pellets you can buy on online for them. They had a completely different diet than koi, so koi pellets would not be enough for them. They are also cold water fish, and do better in colder waters.
I would just go with Gars. Seems like it would be easier IMO.