It took me a while to follow up your Question and the professional answers I got on this was: There is no such evidence that Imam Musa al-Kadhim (as) asked his daughters to marry Sadat only. However he did mention that they must marry people who believe in the Imamat of the 8th Holy Imam I.e. Ali ibn-Musa (as) (because then they are certainly the real followers of the Shia Ithna Ashery faith). There is no doubt that Sadat have the blood of the Holy Prophet (saww) running in thier veins, and that they hold a lofty position in the society from every aspect, but it does not at all indicate Sadat can never marry non Sadat. There is no difference among people but piety. So a Sayyidah can certainly marry a non Sayyid.