In the book of Acts, chapter 9, Paul was given a specific message of what he was to preach so there was not a forgetting of the Gospel or any question of invalidating them.
Paul’s commission came directly from Christ as did that of his other apostles so there is no conflict between the message of the Gospels and what Paul preached, it was “all scripture, inspired by God”.
Exactly! The letters of Paul were not written a priori. This is where both Luther and Calvin went wrong. They are meant ONLY to support, clarify, and reinforce the contents of the Synoptic Gospels. That alone was Paul's mission. They are not stand alone additions, revisions, extensions or new revelations and not to be used out of the context of the Synoptic Gospels.