I think you may have some erroneous information. I am descended from the Yorbas and Avilas, both of whom intermarried with the Serranos, so I, too, have been researching Francisco Serrano along with other descendents.

One thing for certain is that Francisco Serrano is not listed among those in the Portola-Serra Expedition of 1769. Also you seem to think Francisco was born around 1700. The date we have is 1744. He does not appear in the census reports in California until the 1780s-1790s and in various areas, incl. San Diego and Los Angeles as a pobladoro and later as a private soldier.

What seems likely is that he was some relative of de Anza's wife Maria Perez Serrano and lived in Arizpe, Sonora, Mexico and consequently was a member of the de Anza Expedition to Monterrey in the 1770s, then made his way to Los Angeles