Bismihi ta'āla
A Seyyid/Seyyidah can marry a non-Seyyid/Seyyidah. These are only cultural norms, and as long as both sides are Muslim and wajib requirements for marriage are met, the marriage will be valid.
And Allah knows best
Shaykh Dr Zaid Alsalami is an Iraqi born scholar, raised in Australia. He obtained a BA from Al-Mustafa University, Qom, and an MA from the Islamic College in London. He also obtained a PhD from ANU, Canberra. He has written and translated several Islamic texts and also prepared educational videos on Islamic rulings and practices.