I have always been taught the "show me your armpits" cue and in my experience it does create a more stable position. Most Olympic caliber weightlifters also catch with a fairly externally rotated shoulder position, as you can see in this image search.
As for how to get the mobility for an externally rotated snatch position you should hit up mobility drills. Kelly Starrett is great, these two might help:
He's got hundreds more, so if those don't work hit the shoulders from different angles. (Both of those links should be WFS except perhaps for minor profanity? I forget)
Just saw this one from Donnie Thompson about external rotation. Also might help, though it's focused more on getting into a low bar back squat position.
(WFS except for perhaps profanity.)