Cassianus and Germanus are mentioned for the first time in Palladius' "Dialogue on the Life of John Chrysostom", and in Letter 7 of Innocent I presented here. Palladius writes about the Presbyter Germanus and Cassianus being among those who brought the response of John Chrysostom to the Synod of Oak in AD 403. The letter to Innocent I, to which Letter 7 answers, was carried by them to Rome between AD 404 and 406. The Cassianus mentioned here is John Cassian . It is not very probable that Germanus is the same Germanus  who returned from Rome to Africa.
Alexander was the patriarch of Antioch between ca 412 and 417 (or even 421); he tried to heal a schism that resulted from the affair of John Chrysostom. Pope Innocent I consulted on this matter with a Presbyter Cassianus, whose identification with the Cassianus mentioned in the first letter is a hypothesis, which is tentatively accepted in PCBE Gaule 1: Cassianus 2, but rejected by Stewart 1998: 13-15, and Dunn 2015.
Letter 19 refers to "the clerical supporters of Paulinus of Antioch who had left there because of the Melitan schism and relocated to Italy" (Dunn 2015).