You can easily keep fish and plants with a KH of 0. You will never cause the pH to "bottom out," but you can suffocate them (the fish) with too much CO2 since it's easier to dissolve CO2 into low KH water.
Absolutely, depending on the type of fish one is
trying to keep it is easier to "choke" some than others.
So, knowing tank type you are trying to achieve whether
soft or not will determine type of fish selected,
for instance Characins prefer soft to very soft, whereas
Live-Bearers, Loaches etc. prefer it not so soft.
If noting fish gasping or choking at surface, don't freak out
This is what to do, raise the spraybar or lily pipe to break
the waters surface and turn down C02 just a smidgen.
You add 02 that the fish are looking for at the surface and
releases or offgas the C02 which slowly raises the pH.
Finding that balance is not difficult, but it does take some
experience with super soft water.
Overall it is not complicated and a much more primitive
procedure than you would usually see on forums.
02 is your friend.