Since im looking at fairly small fishes, I'm talking about ignoring the 1"/Gallon general rule. I think 3000 small fishes total would be okay for the 2000 Gallon tank, no? 1000 Cardinals, 500 Rasboras, 500 Rummy noses and I'm already at 2k.
I'd try to make it a self sustaining tank, with the highest predator being a large ca/sa cichlid, or maybe a couple pike. I'd start with lots of rcs and try to get large amounts of inverts going in the tank to start with, and then add in a bunch of livebearers and a few convicts. After a month or two of nonstop breeding I'd add the pikes. The tank would be completely live plants too.