Expecting the Radio Shack salesman to know if the wire would work in your
application is like expecting an auto parts counterman to know if heater
hose would work for power steering return line. He should say "No, it
won't work". Of course, I used heater hose for power steering return line
in my '68 Roadrunner. Not perfect, but worked in a pinch(actually I used
it all time I had it, replaced the hose every few months because it
softens and collapses).

It'd be great if Radio Shack hired engineers with years of experience, but
it ain't going to happen. Be nice to the guys that have to work there.
Quite often they do know more about the stuff they sell than the people
they sell it to(not in your case, but you know that now!).

Have a great weekend, please don't take offense at my comments, I don't
intend them to be mean-spirited.

-Bob