Conflicting Instructions

[RichardM]

Quantity of yeast;
I thought this was only important to get he fermentation away quickly thus greatly reducing the chance of any other yeasts or infections getting in.

Surely once underway the yeast will just grow / breed exponentially to meet the sugars available for it to feed on. Would only 0.1g not be enough to ferment 50 gallon if the delayed start did not matter.?

I can't explain the science but there are plenty of calculators to tell you how much yeast to pitch for various beers and various quantities. If it didn't matter they wouldn't exist.