You did not say that you adjusted the stitch length. That is done on the right with the post-type lever that runs in the vertical channel. You may have to loosen it, which you do by unscrewing the outermost circular part. Twenty stitches to an inch would be close to the zero point, which is used for free motion quilting. Move the post lever down toward the bottom of the machine to make the stitch bigger.
You do need the correct walking foot for a slant machine, and specifically, for that machine. There are actually two that work. The Even Flow foot is my second choice, I don't know that the best one even has a particular name.
I find the Singer 301 to be the best all-around quilting machine and my favorite of all time. It is good for piecing, great at line quilting with a walking foot, and the best machine at free motion quilting I've ever used or tested. Be sure to oil it every 8 hours of sewing with exactly one drop of oil at each point indicated in the manual.
Cricket