No rules that I know of, but generally we don't want to be confused.

Fun fact, I was Mistress Morgan (as in Morgan Le Fay) up until I moved to Los Angeles in 2000 and got my first website. There were A LOT of Morgans and Morganas at the time. In an overwhelming lack of creativity I shortened it to a letter. I thought it was literary like e. e. cummings, JD Salanger, etc.

Simone Kross took her last name when she moved to LA not to be confused with Simone Justice, who took her last name in honor of her Studio and differentiating herself from Simone in Chicago.

When I was in NYC there was a Lady M who changed to Lady Magenta to avoid the confusion. Here in Paris there is a Maîtresse M, but There doesn't seem to be any confusion as she is not totally pro.

So it becomes pretty clear that each person wants to be seen as an individual.

I only know of one person who used the last name of my mentor to hers to add confusion with her femdom education.