Someday I may get around to explaining the reasoning on paper but, as I said, it is extremely complex and involves every aspect of cheese-making from pasture to maturing. We have pretty well covered, unless anyone is considering treating their own vells. The effects of rennet will be discussed later. The amount of rennet you use will affect the final cheese so this will depend on the type, or style of cheese you intend to produce. You must measure the quantity accurately for each make.
Salt restricts bacterial growth.
For the starter bacterium to obtain lactose it must absorb it through its cell wall. If you could block its pores it would starve to death. This is more or less what salt does. Different bacteria have different tolerances to salt. Salting too soon can produce a low acid cheese and leave residual lactose available as an energy source to many undesirable organisms.