Leo:
Why is it implausible that some Flavians (which was not just a family but a gens ) converted to Christianity? The church at Rome was huge. The two you mentioned (Titus Flavius Clemens and Flavia Domitilla) were husband and wife. Dio Cassius wrote about them:
"Domitian slew, along with many others, Flavius Clemens the consul, although he was a cousin and married to Flavia Domitilla, who was also a relative of the emperor's. The charge brought against them both was that of atheism ( atheotès ), a charge on which many others who drifted into Jewish ways were condemned. Some of these were put to death, and the rest were at least deprived of their property. Domitilla was merely banished to Pandateria".
MP:
What proof have you got the church was huge ? Can you show me a reference to an archeological site in Rome of considerable size. If there were that many surely they would have found many by now. Your text about DC, doesnt say there were many xians in Rome. It only talks about FC and FD. It also does not mention Jesus or Xians. What group is DC actually talking about ?