What are the chances that a "soft tissue benign neoplasm" on my pinky finger will grow big enough to cause my pinky finger to be dysfunctional?

Up to you. Chance is small as you would notice it and would go to seek for medical attention if you start having more problem with it. It is in your finger so it is easily to be seen.
NO CRYSTAL BALL. You can wait and see or have it removed now. You can have it removed later if it starts bothering you.