I was diagnosed with both. They are just literal explanations of symptoms. With out finding the cause it doesn't even matter.
Fibromyalgia literally means muscle pain throughout the body and small fiber neuropathy/ or polyneuropathy means nerve damage that can also be anywhere in the body. So lots of pain and terrible symptoms, but no solution.
I think that pure small fiber neuropathy is mostly tingling, buzzing, numbness, and nerve pain, where as more invloved neuropathies cause weakness and tremor.
Fibro to me is such an end of the rope diagnosis that doctors use for people who are in terrible pain all over with many ailments who have clean blood work.