How can I get a galvanized pipe that was threaded at the hardware store to screw into the fitting?
I use to run a pipe threader at Home Depot, and my cuts always fit just like the pre-threaded pipe.  Their machine could have been out of adjustment.  If the threads don't look the same as a pre-threaded pipe, and they don't fit the same, something is wrong.

For what you're doing, 1/4" of pipe thread might not provide the strength you need even though it doesn't have to be water tight.  Take it back and show them the problem.