I'd have to see the exact minute mark of the lesson you are talking about to better clarify, but I think what you are referring to is the fact that Facebook's auction works based on impressions, even if our objective is conversions. But if you are going for conversions, you should definitely be optimizing for conversions. There have been a few changes to the labeling system FB uses between 4.0 and now so it's just a little confusing without the lesson as a reference. If you want more precise clarification please link to the video with the minute mark. Regardless, conversions is what you should be optimized for in your case.

I answered the second part of your question about IG/FB story placements on today's coaching call. Let me know if you need further clarification.