What ever your project is, the leather doesn't care what twist the thread is.
Hand sewing is not hard on the thread, some sewing machines will
not sew with the wrong twist thread. I would not use nylon I would use
the polyester or the linen thread. You might consider using a linen cord around
the number 3 size numbers 5 and 7 are way to heavy for hand sewing in
my opinion but there are folks that use them.
I don't know what to say about the awl size. If you don't like what is
available make your own from a nail. I would use the 6 penny finishing nail
my self but that is up to you. Get a propane torch, a hammer, a file and
something to forge the awl on preferably an anvil. Don't for get to heat treat
your new awl. It makes no difference if you heated the nail in the process of
forming. Use a pair of plires and hold the nail, heat the nail to a straw color
the full length of the nail. You can quench the nail in oil if want to that makes
it a little harder.
This will allow you to make all of the awls you want in what ever size you want.
And there will not be any sticker shock from this method and as the one
post said it is in the hands of the craftsman not completely in the tool.
Practice, practice, practice. Have a nice day.