I don't like the sentence fragment rule. I think it can be quite effective if used correctly. I also think an effective sentence fragment is discernable upon inspection, credentials are unneccessary.

Anybody who needs for you to be a best selling author before they know if your use of a sentence fragment is effective has no business giving writing advice.

Don't use a sentence fragment unless you know what you are doing. If you can't give a good reason for doing it, you shouldn't do it. That is what the advice should be. That advice is same for most rules in writing.

Arudahti Roy, in her first novel ever, uses a sentence fragment that I can remember today, five years after reading it. It was that powerful.

"She was thirty years old. A viable, diable age."

All I could think when I read it was, 'I wish I had written that'.