Hi Ebell,
I've just taught a ruler work class this morning. We had a few different machines in the class. And some of the students bought ruler feet off ebay, and we couldn't get them to work. They were supposed to be "low shank," but when we compared them with the Westalee "low shank," they were definitely taller, and squoze the fabric, too tight.
In another class, we had a student with a ruler foot that was chromed plastic. It, too was too high. So we went to grind it down a bit, which is when we discovered that it was plastic. Yikes!
It really is worth getting the Westalee foot. They're $48 and come with the 12" arc ruler. But you get a good quality foot, and it will work like you want.
I've found that polyester thread behaves better than cotton thread when doing free-motion or ruler work. Probably because it has some stretch, and cotton doesn't.