So to clarify, playing a character that's literally a carbon copy of a servant is allowed, but playing a character that is figuratively a carbon copy is not?
Playing the canon character itself is fine. Copying a canon character's theme and turning it into your special OC isn't.
And on the other skill, Book of Five Rings is a skill that lets him hold his own against NPs provided he has prep time.
It ties into his real-life historical habit of exploiting weaknesses in enemy techniques and weapons, and also he wrote a book of how if he caught you doing this he'd kick your ass.
His combat stats being higher than Kojiro's makes sense because, historically, he beat Kojiro in two hits with a stick.
The high strength especially makes sense. Katanas are heavy swords, and Musashi used his one-handed to develop his dual-wielding style. Moreover, he's historically described as being physically very large. I recall his height being listed as something like six feet, which was tall for a Westerner, let alone a Japanese.
I want an actual codification. What do you mean "hold his own"? If I give him prep time can he dodge Gae Bolg or smash through Kavacha and Kundala? What does it mean?
Do you know how many Servants outside of Grand Order have A rank STR at base, at base meaning "not counting things like Mad Enhancement"? The answer is one, and she has a dragon roughly the size of the Fuyuki Park inside of her.