BTW - the darning feet I have that are original Singer are rather short. They actually do not come into contact with the bed of the machine when the foot is down. They work best when you can keep a good tension on the item you are sewing, either with firm hand pressure or in a hoop.
With my viking, it recommends decreasing the foot pressure, but that machine has an easy dial to turn. It's as easy to change the pressure as it is the tension.
I did a bit of FMQ with my White 670. The hoping foot would not fit so I used a darning foot and released the pressure. After a bit I went back to the 15 that the hopping foot fit well.