Excepting specialty shots which require a steeper attack angle or a curve lefty or right, etc. The twist is the same.
If you find your measure to the ball with a good address procedure the amount of forward bend will be determined by the length of your club. I would caution against changing how you stand to the ball from club to club, and instead find a way to measure to the ball the same way each time. This eliminates that part of the equation.
As Sacto said, loosing the angles you set at address can be more difficult with a short iron as there is more forward bend.
It sounds though, like your MGS swing is solid, and I suspect that your club face is clsoing too much for the balance of your clubs with your irons. Again, I would caution against trying to consciously trying to employ shoulder rotation. With a proper MGS set and twist, you will get the rotation you need to have at the appropriate time. Remember, MGS wants you to make no INTENTIONAL/ conscious attempt to rotate, bump the hips, or manipulate the fore arms or wrists as you swing.
I made a video on balancing the club head and ALLOWING the clubface to square. I also made another one that talks about a very precise way to set your measure to the ball EVERY time.
Watch these videos, and see if you are better able to control the curve of your ball by employing these techniques:
Measure to the ball: Arm Hang
And then: Allowing the Club To Square
I can guarantee if you follow the advice in these two videos, you will find you have an uncanny ability to control the curve of your ball. Whats more, you will be doing it without changing your swing biometrics at all - Everybody has a different way that thier arm moves through the impact zone - it's simple anatomy. Also, each club, due to weight, length balance etc will square differently when swung with an arm swing that does not consciously manipulate or rotate while swing.
MGS facilitates a very consistent and repeatable arm swing - based on our body's natural motor neural pathways and bio mechanics that have been in place for a long time. Take advantage of this simplicity by discovering how your club face squares for your swing.
Let us know how you did!