So if I'm understanding you correctly you wouldn't need a pack for basic but for level 2 you would, then for level 3 you can take the pack off again?
I have to say I hope this isn't the case for the Mimban Stormtrooper as those with packs are only seen very briefly. The only 2 clearly seen onscreen Mimban Stormtroopers (the ones who throw Han to "the beast"), the promotional materials and even action figures all have no pack.
Unlike the sandie I'd argue the pack is not an iconic part of the armour and having it as an optional item is more in keeping with the movie.
Just my opinion.