Pokémon B/W. How's the adventure going so far?

[Wes]

Been procrastinating on challenging the Elite 4, went out and caught the -ion trio, mostly stumbled upon them by accident while exploring. I caught terrakion with a premier ball

Anyway, should I leave a spot open in my party for Zekrom when I challenge the E4?

There's a chance in between the E4 and the legendary battle to drop someone off in the PC, and when you catch them and have 6 pokemon, the game will let you swap someone in your team out right on the spot. So you can use 6, its fine.