Is the person pinning a Soldier supposed to be equal or higher rank than the rank you are getting promoted to (other than family)?

I have been led to believe it is up to the person being promoted.

There was a Senior Chief that was promoted to Master Chief on my last ship. He had the most senior and most junior enlisted pin him.

I went round and round with my 1SG about this. When I made SSG, I chose my most challenging soldier to pin me. He had challenged me in every aspect of Leadership and Mentorship, overcoming marriage, drinking, and work problems. I molded him into an outstanding soldier, husband and father, as well as a dedicated NCO later on down the road. Without his "issues" I would not have been able to develop myself to the Leader I am now in respect to dealing with Soldiers and their problems. Eventually, 1SG trumped me, and my Rater ended up pinning me at the last minute. On a good note, I did get to thank him back when he asked me to welcome him to the Corps when he made cutoff for SGT, and I was able to proudly share those sentiments with him and all our peers.