If you're getting a Westalee foot for your machine, you'll need a low shank foot. It will only work with low shank rulers -- 1/8" thick. Taj and Elvira are thicker for use on longarms or high shank domestic machines. They won't work on your machine because the clearance at the back of the foot isn't high enough. I know one woman who uses the taller rulers, and just makes sure they don't go to the back of the foot.
The presser foot on your machine can be turned side to side to be centered with the needle. But then you might have to turn it back to use your regular foot.
I do free-motion and ruler work on my little Bernina with the feed dogs up, and with a Westalee ruler foot. With the feed dogs up, I get a rhythm that helps me keep my stitches more even. With your Singer, you would loosen the presser foot tension, which is the knob on the top of the machine, above the needle. Remember, "righty tighty, lefty loosey."