So long as Zard can't turn Boosts into fire, or any other special energy for that matter, it won't be broken. HOPEFULLY, they do this and make him only be able to use basic energy cards for its attack. If they do this, he's definitely NOT broken.
And prime, the game is not just ex-mon. How many ex pokemon do we see in decks? There are 2 Rayquazza in Blaziken, and in niniken there aren't any at all lol. There's still plenty of pokemon that aren't ex that work. Sceptile, salamence, victreebell. Sure, some are mixed with some ex's, but they definitely need the non-ex to work. When these come out, THEN we'll see how the format is affected.