Mike-
Question 2: There are many manufacturers of shirts that will work with dyesub technology. The higher the amount of poly the more vivid the colors are produced post-imaging.
50/50 is the lowest poly content I would use as it gives a vintage / washed out look post image.
Anything less than 50% isn't commercially sellable I.M.O, unless this is your market
Hope this helps,
Jae