Does tracking Google Snack Pack/Videos element count as additional terms from my plan limit?

No. When you choose to track Google Snack Pack/Videos and Organic for the same search term,

PRT will track both the organic results and the Snack Pack/Videos results of your URL.

However, they will only count as one term out of your plan limit.

Organic plus any number of searched elements tracked for the same term-URL combination will count as only 1 term out of your quota.