I would just cast on with the smaller needle and the normal number of stitches.
As for the slipping..I would also probably always slip the same purlwise with yarn in back and then move it as necessary. But, you could also slip purlwise with the yarn in front for purl on purl rows and in the back on knit rows. It's really not critical to the pattern as far as I can tell. If it was the designer would specify.