Some people claim that Imam Musa Al-Kadhim (as) did not marry his daughters because there were no Sayyids available, using this as evidence that a Sayyida cannot marry a non-Sayyid. Is this true?
This claim is not true. Great Sadat scholars including Maraaje' of Taqleed, have allowed marriage of their daughters with non Sayyed Momins. The claim mentioned in your question came from certain culture with no concrete evidence in religion.
'Wassalam.