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

*Since for Snack Pack a location is required, only adding also Local Organic (and not Organic) with the Snack Pack will count as one term.