[Guardian and impact]

If I have Royal Guards within range to Guardian Dark Troopers am I correct that if I have priority I can guardian before impact but if my opponent has priority he can convert impact before guardian? Or is impact not a card keyword so that takes president and the impact turns a hit into a crit before guardian? 

[Cover Keyword]

If I have a unit with the cover keyword like bikes and it is engaged with my opponent's unit and he uses Call Me Captain so Rex can fire support into the melee would I get to cancel 1 hit as it states if defending against an attack with at least one ranged weapon?

[Team and Grand Army?]

There is no mention of Team games or Grand Army games are these formats not official anymore?