For exercises like fly's, the natural movement of bringing your arms back together in the movement, prevents "pinching" the shoulders together. In other words, just let your shoulders assume a natural position during this movement; dont focus on them too much. However, for a movement like benching, then yes you want to "pinch" your shoulders back into the bench.
As for movement speed. With certain exercises you will sacrifice certain things. In the case of fly's, always go slow. On the upsing part of the movement, you want to try and contract your chest at the same time. This technique is more advanced though and may be hard at first. Other movements like pause reps on bench and then exploding upwards will work the other fibres.