A minion group has number of skills ranks equal to number - 1

So a group of 4 stormtroopers has 3 ranks in ranged-heavy.

Does this decrease as their numbers are removed via combat? (if they have 3 ranks in round one, and the PCs eliminate two during round 1 - what is their skill check in round 2?)

My assumption is it DOES change as the number of minions change.

Correct. Your yellow dice will downgrade to green as the number of minions in the pack drops.

I would agree with your assessment. The entire point of the rule is that minions are more formidable in number and since they act in concert, a single GM "action" represents multiple shots or actions by the minion group. Therefore when there are less minions, there is less "help" to go around for the minions.

A group of four stormtroopers will have Ranged Heavy (3), kill two, and they will be reduced to Ranged Heavy (1).

You are correct, the skill ranks lower as the minions get picked off.

I like using minion groups in this way, as it allows you to mitigate the incoming damage on your PC's. Instead of having a potential 4 heavy blaster shots ripping into your party, you would have one rather precise one. Takes away some of the chaos, especially in a case when you want to feel like you have kicked up the tension, but don't want to risk killing your PC's unduly.

Oh, I played this wrong then...I gave EACH minion an action, with ranks equal to the number of minions present...no wonder the stormtroopers were better than I thought they should be!! But, I did at least reduce the number of green die used when they were each killed...

Thanks for the feedback. Considering the level of abstraction it makes sense that you would be 'hit more often' with 4 troopers shooting at you rather than two. A hail of blaster fire as a single roll as opposed to Saga edition where every roll is essentially a 'pull of the trigger'.