I spoke with the Superior Threads home office about my experience today. The So Fine prewounds that I used are a new product, and they will look into improving them.
Their theory about why the So Fine thread that I wound myself puckered with the YLI monofilament is the texture that they put on the So Fine thread. A textured thread does not play well with a slick thread like a monofilament. The DMC embroidery thread that I usually use is slick, and that is why it worked when the So Fine didn't. They think their Bottom Line thread would work with the monofilament.
Interesting... if nothing else, I'm getting an education!