Are you saying that Paul laid the foundation for purgatory, annalex? He clearly states that he laid the foundation according to the grace of God that was given him. Wouldn’t that make him of greater importance to the Catholic Church than Peter?
No I am not saying that. Certainly St. Paul is not saying that: "neither he that planteth is any thing, nor he that watereth; but God that giveth the increase." Since you ask, Jesus laid the foundation for the Purgatory by His death and Resurrection on the cross. St. Paul speaks of the foundation of the faith in Christ that he provided by his priesthood.
Jews had taught about Purgatory before Paul was ever born and understood there was a place where people would suffer for what they’d done while alive even though they were not damned to Hell. Paul is explaining that an existing understanding is correct and still in place under the New Covenant, not adding anything new at all.