That's true , as long as we realise that this view relies upon past proof and hindsight. We see the NT on the same level as the old now, only because some people canonised it.
But if you were a Jewish christian living in the time of Paul, you probably wouldn't have considered his letters as weighty or as much 'scripture' as the psalms, proverbs, Isaiah etc.
Paul rarely quotes his own 'scripture'. If he considered his own writings to be absolute authoritative scripture, he would have said to the Collosians, "as scripture says in 1 Corinthians.. etc etc". But he didn't, he appealed to their scripture - the old testament. I don't believe Paul considered his own letters as mystical, prophetic or weighty as Isaiah or Jeremiah or Genesis, as afterall they were mostly practical instructions to the churches under his authority. What is important are the prophetic and gospel truths that Paul communicates in those letters. I don't think when Paul said things like "send me some money", that is particularly prophetic or worthy of a 'name removed from the book of life' penalty for changing it.
The warning in revelation only appears to apply to prophecy.
Yes Paul sought God's will, but God may or may not have spoken about something. In which case he would give his best personal judgement. Paul knew the difference between a word actually from God, and a word from himself. He knew when he was speaking with the authority he had as an apostle, and when he was speaking as a frail human being.