The 10 gallon rule

I'm a newbie to Goldfish care. I'm seeing over & over the
fecommendation of 1 fish per 10 gallons of tank to start with. To me
this seems excessive for small juveniles (although I understand the
rationale for older fish).

a) Is that assumption wrong - that even small juveniles need that much
tank?

b) just how fast do they grow i.e. how many months or years should it
take for say a tiny 2" GF baby to require such space?

c) do all you serious GF ppl not even bother with the small fish &
average stores & hunt around for, only taking home, perfect fist sized
or better specimins?