Okay... I'm sorry, but that's not a 350 gallon tank; more like 350 litres. My 5'x2'x2' is only 150 US gallons...
It's not a matter of what's 'allowed', it's a matter of giving your fish what they need to live happy, healthy lives.
Fish have evolved over thousands of years to survive in certain environments, and keeping them in something different is going to have huge implications for their welfare.
If your water is hard, that's going to affect your soft water fish, and if your water's soft, that's going to affect the hard water ones. Harder water fish do
extremely badly in soft water; much worse than soft water fish in hard water; their bodies have to work so much harder to expel all the excess water they pick up through osmosis that it leads to stress related illnesses, such as bloat, and eventually kidney and liver failure.
Perhaps it's just me, but I'm not finding any of this all that funny