Casting and what flies/tactics are two separate issues IMO. The former I would leave to an instructor. Asking a random angler you may pick up as many bad casting habits as good ones. This ties in to your last idea about new fly lines - casting in a crumpled heap would be down to casting not the line i would have thought.
As for flies/tactics, I see nothing wrong with a polite enquiry to an angler, and think most would respond as you would hope. It is part of how we all learn, and also part of fishing as a social not entirely solitary activity. You will always get the odd curmudgeon but that's life
A lot of fisheries offer affiliated tuition of some description, or there is bound to be someeone on here wherever you live who can help or point you in the right direction.