To answer your first question, at least, a minion group only has one "turn" per round, and the dice pools for their skill checks are going to be based on the number and type of minions in the group.
For example, a minion group of three stormtroopers has a Ranged: Heavy skill of 2, which translates to a dice pool of two proficiency dice and one ability die. If one of those stormtroopers is killed, however, (represented by the minion group taking 5 or more wounds) the dice pool drops to one proficiency die, and two ability dice. Essentially, the Ranged: Heavy skill for a minion group is based on the base minion's ability score (3) and every additional minion in that group upgrades a die in the pool.
If you had four stormtroopers in a minion group, the Ranged: Heavy dice pool would be three proficiency dice.
If the minions are not in a minion group, (subject to how the GM wants to use them) you would make a roll for every single minion at their base ability. For four stormtrooper minions that are not in a minion group, that would mean four minion "turns" and three ability dice for each roll. Place those four minions in a minion group, and now they all act on one turn but have a significantly upgraded dice pool.
I hope that makes sense.