There are a couple of possibilities. Perhaps in the small size you can continue in pattern without the need to use stocknette at the shoulders. I think the reason for the switch to stockinette is because it becomes difficult to maintain the pattern while you're doing the shoulder shaping and decreases. Why this should happen on the front and not the back shoulder, isn't clear to me. It probably doesn't matter because there are epaulettes which will cover the shoulder seam and last few rows of the shoulder (photo
). The stockinette may even make the shoulder lie flatter (without the bulk of the pattern st.) under the epaulette.
So, I would continue with the 16 rows in pattern and use the stockinette at the last few rows of shoulder shaping
when it becomes difficult to maintain pattern.