“Necessary”, by definition, incorporates “sufficient.”
So far as cessationism is concerned, I am not one. I do believe in the continuing work of the Holy Spirit, but I do not believe in an extension of the scriptures, since the central point of history was the death, burial, resurrection, and ascension of Jesus, and the witnesses of His ministry attained their place within NT scripture by virtue of their having been a witness, or an amanuensis of a witness, of the resurrection.
Therefore, as you say, no utterance truly from the Holy Spirit will contradict the already given revelation of God as found in the scripture.
Nope. As an example, let's take your car.
An engine is necessary for your car to work as a car, but it is not sufficient -- you also need gas, tires, and so on.
In the case of Scripture, I agree with you that it's "necessary". Even according to Scripture, however, it is not sufficient.