To clarify at the outset regarding the way the gospel accounts all speak of Jesus being the Son of God:-
The elderly apostle John, who wrote his gospel account last, was relating what John the Baptist said about Jesus, at his baptism: "And I saw and bare record that this is the Son of God" (John 1:34). This was stated before Jesus called Andrew and the others to be his apostles. In John chapter 1 the second person to call Jesus the Son of God is Nathaniel (not Andrew). Verse 49 shows this clearly.
The gospel account that puts particular stress on Jesus being the Son of God is that of Mark. His opening verse sets forth this fundamental truth: "The beginning of the gospel of Jesus Christ, the Son of God."
In Luke's gospel account he records the angel telling the virgin Mary the astounding news that the miracle baby she will give birth to "shall be called the Son of the Highest" (Luke 1:32). Well, the Highest is God, so (clearly) Jesus will be called the Son of God. That seems to be the first statement declaring Jesus to be the Son of God. John the Baptist's declaration seems to the the second time, with Nathaniel's the third, and Peter's some time later.
Matthew's gospel records Simon Peter (Cephas) declaring Jesus to be the Christ, the Son of the living God, and Jesus saying that he was given that revelation by God the Father (Matthew 16:16-17). In no way does that conflict with John 1:34 because the statements are not being written in chronological order. This answers your first query.
Your second query is about Peter being called Cephas in both Matthew and John's accounts. However, it is unclear what you are asking about that. If you are querying the timing of when Jesus gave Peter that name, then all that needs to be said is that that has no bearing whatever on your first question, and that the gospels are not biographies, so chronological order of events, and when something was said to another person, may be in a different sequence in different gospel accounts.