Hi there - good questions…
Generally, they are “more common” in fibro than without, but I haven’t found numbers.
Important thing to watch: Lipomas with pain may seem to be fibromyalgia, but can actually be Dercum’s disease: Dercum’s Disease – A Mimic of Fibromyalgia | Dercum's Disease Research
And personally: I have one, which my wife saw, not pre-fibro. It doesn’t cause me pain, and I “had it taken care” of by asking my dermatologist and she said it’s OK to leave it as long as it doesn’t change: so that’s taken care of 
. Got enough going on, don’t need any extra scars that’ll cause more problems. (As far as cutting goes I have a probably harmless tumour in my spine that needs checking once a year…)