What statements did Paul make about his apostleship in his greetings to the churches?
Paul did not hide the fact that God had called him to this special office. In fact he was forced at times to defend himself against those who denied it. This is evident even in his opening lines to so many of the churches and men to which he wrote. Here is a collage of all those introductions:
"Paul, a servant of Jesus Christ, called to be an apostle of Jesus Christ through the will of God (not from men, nor through man, but through Jesus Christ and God the Father who raised Him from the dead) , by the commandment of God our Saviour and the Lord Jesus Christ, our hope, according to the promise of life which is in Christ Jesus, according to the faith of God's elect and the acknowledging of the truth which is according to godliness..."
What more could he say? This job was his. He was not ashamed of it. It had been handed to him from Heaven, and he intended to fulfill his calling. No other men but the "twelve" and Paul made such statements, nor could they make them.
Since Paul considers himself an apostle to the Gentiles, can he be a part of the original twelve ? Were they not apostles to the Jews?
Paul was a Jew. He was a "Hebrew of Hebrews." He first went to the Jews in the cities he visited. But while the twelve stayed in Jerusalem at first, Paul was being sent to the nations of his day. In Christ there is no Jew or Gentile. That was the meaning of Paul's life, the breaking down of the barrier between us.
There is one church. Just as there is one tree, Israel, into which Gentile "branches" were grafted, so there is one people of God, the church, which began with Jews, but will include people of all tongues.
Paul's mission was exactly the same as that of the other apostles: to call out a people for His Name. That mission has been handed by the apostles to us in our day.
What was Paul's personal view of his own ranking?
In I Corinthians 15, Paul lists hundreds of people who saw Jesus before he did. He then says that he was the last to have seen Christ, thus being the last witness of the resurrection. This witnessing of the risen Jesus seems to have been in Paul's mind a qualification for being an apostle, as he cries out in chapter 9, "Am I not an apostle... have I not seen the Lord?"
He suggests also in chapter 15 that he is to be considered the least of the apostles, due to his treatment of believers before his conversion. But though last and least, there is no doubt in his mind that he is an apostle. I think that those today who must search their hearts to see if indeed they are apostles have been seriously misled. Apostolic calling carried more certainty than that!