There is no rule of thumb on this. It's which feels best in your hands and delivers the accuracy you require. Having said that most people with small hands prefer the small; medium hands, the medium, etc. Get out there and shoot the sucker, mix the grips arounds a little, and you'll know in very short time which is best for you. For my short fat fingers, I prefer the small.
You asked about shooting drills with your v1. There are an infinite variety of drills that you can use... That said, the most important, too me, is becoming intimately familiar with unholstering and holstering your weapon. As in, you need to get to the point where you can do this seamlessly and smoothly in either direction. This includes getting your dang shirt out of the way!
Also, in HKPro's MP5 classes, he makes us draw from a DA state when we have to transition to sidearm. If you are not used to making that first DA shot, you should practice!
The different backstraps will not only change the "feel" of the pistol in your hand, but, just as importantly (if not even more so), they will affect your trigger reach. That is the aspect I would focus upon in determining the most appropriate backstrap for you.
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