I think Shanks having multiple styles has a high probability
Mihawk and Zoro are dedicated to swordsmanship. Shanks on the other hand wants to be PK.
Xebec had multiple styles. Roger did as well, using a gun in chapter 0.
This doesn’t mean Shanks isn’t a swordsman. Rather that swordsmanship is just one tool in his arsenal.
Mihawk is Shanks’s rival, and they are close. But the plot clearly gives one greater importance and it isn’t Mihawk.