Number 9 shot is THE correct size for all skeet shooting it's the only shot size most skeet shooters use.
I don't pattern my guns except on the skeet range. If I'm right I get smoke hanging in the air from the shot, if I'm centering the targets and not getting it I know something isn't right. I use Briley choke tubes in all my guns. In .410 and 28 gauge I use those marked SKEET and for 20 gauge I use CYLINDER in the lower barrel and SKEET in the upper barrel. For what little 12 gauge shooting I do and that's very little I prefer CYLINDER tubes.
From ALL stations other than station 8 it is 21 yards from the station to the center pole crossing point.