” Sephardim have no need for, or even a concept of, Heter Meah Rabbonim. Since they have no prohibition against polygamy in the first place. ”
While it holds true for Yemenite’s and even Sephardim, however the minhag of Sephardim is not to, as per haRav Ovadia Yosef the נוסח הכתובה of Sefardim is as follows:
“ולא ישא ולא ישדך ולא יקדש שום אשה אחרת עליה כי אם ברשות בית דין הצדק”
See יביע אומר ח’ אה”ע ב’.
However, in some cases permission of a Bet din will allow someone to marry another woman.