Carefully culled and raised Japanese koi in Japan mud ponds in the hands of a Japanese breeder can and will hit 2.5'-3' in under 10 years. In our hands, I'd not expect it. Yeah, they are expensive fish!
You can get a 1.5'-2' RTC from The Fish Place (or Pets Plus in Lockport, or Markheim's pets) for $50-$100, or so it used to be.
Your friend's RTC is the biggest I've heard of raised in captivity. This is quite rare, I think, hence, I'd not count on it. Taksan (a former member here) kept a pair in 15,000 gal for 27 years and they maxed out at 4'.
TSN would be better with koi that it cannot fit in its mouth by any stretch of imagination. RTC is known to still try. Still a gamble. But TSN will not break 2.5'-3' as what we get is refuse culls from food fish farming or cheaply raised TSN from SE Asia.
2 Koi is just a start, a seeding, or a catalyst. It's common knowledge that one can't stop buying koi until there is more koi mass in the pond than water
I think so. I lived in Rochester NY for 10 years and we had a 12,000 gal koi pond, 6' deep, did ok in winters with just a deep water fish tank bubbler.
I am not aware of the Chinese high fin barbs/sharks/loaches needing salt in their water.
Sturgeon however are rarely all freshwater. Most are anadromous, meaning they enter freshwater only to spawn, like salmon. Some are catadromous, entering marine water to spawn, like eels.
Some get landlocked.