As long as you're mainly B's you're fine, a C or two won't drag you down. They'll look more at your personal statement so make sure to work on that and really show you're passionate about commerce. Even if you get mediocre grades you'd just be offered conditional entry (I'm not too sure if they even offer unconditional for commerce). Don't worry the gateway acceptance rates are pretty high and most people get in, its just medicine and optom that are selective.
And co-op is definitely a lot harder to get into since the overall spots are limited as well. But likewise if you show a strong interest and meet the 90 atar threshold (I think) you might be offered an interview so don't worry too much^^