Second that on nylon with what I hope is a wake up call rather than is taken as an insult. After all, the only really stupid question is the one you didn't ask.
Have you ever sailed? How could you possibly want stretchy line for sail control?
Your concern about "shock loading" the halyard is so far beyond reality that I'm at a loss as to how you think you might correctly rig any boat.
It is true that some classic fleets suffered spar breakage when there was a mass conversion from natural fibre to dacron sheets but I can't imagine that you're old enough to remember that because if you were, you'd already know the answer.
The question displays such profound lack of understanding of the basics of sail control that I'd rather not answer your questions directly without some insight into your experience. It's possible, for example, that you've been an AB on a tug or ferry or something and, while used to line handling, don't know about sail control. It's even possible that you've been a crew on some boat somewhere but did not learn the why's of what you were doing.
When you set a sail, you want to adjust it for the conditions and then not require constant readjustment. Assume for the moment that you're looking at how to set up a marconi (triangular and attached to the mast and most often boom) sail. You may sail enough to get the idea how in heavier weather flattening the sail out by increased luff and foot tension. If the halyard and/or outhaul creep, you'll just have the sail bagging off depowered and in a poor shape. So, you use dacron or perhaps even more modern really no stretch exotics) to put AND KEEP the pounds of force you want.
Same with the sheet. You want to haul on the mainsheet such that the sail is where you want it and stays there. If you need to ease or trim the sail due to wind shift or a little gust or whatever, you want to control that rather than have the accidents of stretch for a given line diameter as modified by the mechanical advantage of the sheeting tackle.
There is such a thing as shock-loading a sheet if you don't handle it correctly during a gybe. Rather than rely on sheet stretch that destroys your ability to trim the sail correctly, learning elementary line handling does the trick.
Please give an indication of the boat and her rerigging needs. There are general standards for suitable sizes for a given sail area. But more than this, please give a hint as to what sail-knowledge resourses you have at hand - books, mentors, wise old fellow with pipe and cool hat, whatever - so that people here can give you a notion of where to look for further guidance.
G'luck
Ian
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