Well, to me, for the Catholic Church to say that all Catholics must believe that, e.g., Vatican I was an ecumenical council, would be a bit too ultra-montanist (or if you will, a little too "papist"
) for my taste.
But, but, to keep it in perspective, that would really not be nearly as bad as if the Catholic Church were to say that all Catholics must believe that, e.g., Ineffabilis Deus
(the 1854 declaration that Mary was immaculately conceived) was an ex cathedra
"Wherein" the astute reader may well ask, "lies so great a difference in these two cases?"
The difference, the big difference, quite simply, is that the Catholic Church has never dogmatically defined that ecumenical councils are infallible, but has
dogmatically defined that ex cathedra
statements by the pope are infallible. In other words, if the Catholic Church were to say "All Catholics must believe that Ineffabilis Deus
was an ex cathedra
statement", it would immediately follow that all Catholics would be required to believe that Pope Pius IX exercised infallibility on December 8, 1854.