CD when I use my 127 or 15 to mend I also use a HC. I have a HC on both the 127 and the 15 so if I need to do FM or applique' work I can go very slow with one hand on the work and one hand on the HC. I have a lot more control with the HC than I do with the treadle. Having a HC on your treadle is a little noisey when you are not using the HC just treadling, but worth it for mending and FM work. I hem all my jeans with my Singer 127 Sphinx. I do FM and applique' with my Singer 15 because it drops its feed dogs. They are both treadles/HCs.
Good luck
Nancy