No - The 6-second spot would run as a shorter ad in the 15-second spot. The cost and inventory remain the same as the 15-second spot.
You can run an 11-second ad on a 15-second inventory.
We recommend a shorter duration (such as 6, seconds, 11 seconds, and 15 seconds) for Outstream with captions, as it is an in-feed asset that is scrollable.