Are STO novels considered canon to the rest of the novelverse?
No such thing. "Canon" doesn't mean "continuity"; it means "the stuff made by the series' creators/owners," period. So the word only applies to film and television Trek productions, not to any licensed tie-ins. What the novels have is simply a continuity. And that continuity has never been meant to include every novel; the majority of novels in recent years have been part of it, but there have always been others that stood apart. The one and only STO novel, The Needs of the Many
, was written to be part of the STO continuity, not the novelverse.