A Skylark plan can be found in "Signs and Portents 15", "Rise of the Lightbringer" article.
And yes, both ships are canon since they were both on screen in the show (though the name "Skylark" is a fan invention).
But I have written it before, Mongoose needs to do a lot more ship plans - best in really nice versions (Anyone who has the Serenety RPG should look at those ship plans - that is what I am talking about) also usable as good-looking maps for miniature gaming when enlarged with a color copy machine...
I'd buy such a supplement.