If your snake is eating rosy reds it will eat guppies. I've never heard of a garter distinguishing between fish.
Any fish = Food

Have you tried the worm chunks in with the fish?
The more hungry a garter gets the less picky they are.
What size container are you using when you feed? You are container feeding, right?

Try this. Put guppies in with rosy reds and watch what happens.
Make sure the guppies out number the rosies. Your little scrub will eat whatever it grabs.