Too little will allow nitrogen to be absorbed into the weld pool causing weaknesses in the weld and parent metal, a bit less can cause sub surface porosity that you won’t see unless you dress the top off the weld and even less will cause severe porosity, as said above too much will cause turbulence which will have raw same effect,
We always advise between 12 and 15 litres per minute as being optimum although there will be loads come on and say they’ve welded with 8 for the past 20 years and never had a problem. At the end of the day it will depend on how critical your welds are
NoCan different flow effect the heat