i dont get this statement, you first say Christians dont decide scripture, then your next statement is Christians get to decide scripture, if the spirit tells them to.
So let's back up. Our Old Testament was already fully established as Scripture by Christ, but as found in the Hebrew Tanakh. That had three divisions: (1) Torah (the Law of Moses), (2) Neviim (the Prophets), and (3) Ketuvim (the Psalms or Writings). All the apostles were familiar with these Scriptures. And copies of copies of copies were made faithfully by the early Christians.
Then the Gospels and Acts were circulated among the churches, and again copies of copies were made and handed down. Then the epistles of Paul were circulated among the churches, and Peter, speaking by divine inspiration, classified all of them among the OT Scriptures. Again copies of copies of copies were made.
And thus all the original inspired writings were copied and circulated among the early churches. As a result between the 1st and 2nd century, a Syriac translation of all the Scriptures (OT & NT) came into existence and these were then copied and circulated among all the Aramaic and Syriac speaking churches. The Peshitta has the same NT books as we have.
does this mean i can start writing my own scriptures, claim the spirit told me to, and then demand they be added to the bible?
As you can see, this did not allow anyone to manufacture their own Scriptures, and they dared not do so, since the early Christians had tremendous reverence for the Word of God. The majority of the earliest Christians were Jews, who had a deep respect for the Tanakh and the Torah.
At the same time Gnostic heretics began corrupting the Scriptures, but faithful Christians rejected them. As a result many survived through lack of use.
the Tim passage about all scripture being breathed by the Most High was made at the time the LXX was accepted, hard to get around that one.
That is incorrect. Palestinian Jews did not use or need the LXX, which had been translated for Alexandrian and Babylonian Jews outside of Palestine. The claim that the LXX is largely quoted in the New Testament is false. There is some resemblance in about 10% of the quotations, but there are also major issues with corruption in the LXX. So do you think the Holy Spirit would approve a corrupt bible?