They are allow to double trap, I believe, but rarely see a single trap.
Actually, a square head mainsail, no matter the material would be good. Seems the Pentex and mylar are not much different in price than the Dacron.
Problem with aluminum for poles is they come in 12' then 24' section. Personally, I think a 20' boat needs a 15' pole to be effective. In that case, alum costs more than carbon or fiberglass.. Easy enough to get an old windsurfer mast