This trifling tiefling scales in power with the number of lands your opponent controls, so it'll naturally hit the 7-power threshold at some point unless opponents want to intentionally sandbag lands
For example, Pokmon 151 features promo Mew and Mewtwo cards that, while part of the Black Star Promos, are often tucked into 151 collections
If you have any concern about whether a card is allowed, please confer with the Head Judge of the event before using the card
Green decks will be able to power him out reasonably often, but there will also be times where you just can't get enough power on the board
However, this encourages players to try new cards and combinations
It's this last category that has absolutely blown up in the last year