Can you only obtain the "new" character classes secondary ability through Rux?

It's more complicated than that.  So far, the abilities Rux has offered have been only bling potato mine, super guided ball, and arcane lotus.  One of those is for cactus, not a new character.  Also, the bling mine is only cosmetically different, unlike the others.  Like I said, you posed a good question.  The sticker packs offerings have gotten hopelessly complex, you hardly know what pack might get you what items anymore.  I wonder if some of the new abilities might at some point be obtainable from the character "fun packs" that are offered with the hero showcases.  So far, I think they have been Rux exclusives.