From what they’ve shown, not only can they switch on the fly but activate abilities and supers practically back to back. From what I vaguely recall, the wording sounds like it works like a mana pool. Things deplete X amount of energy per use and Guardians store a varying amount, based on their personal strength. They just don’t let us operate like that in the game cause it’d be bananas. Watch any cut scene and you’ll see the game vs the lore is very different. Never seen a shield in any cutscene or web comic. Bullets make full contact.