I'm a month away from starting my 18th year carrying a Sig on duty. Time flies. They've really grown to feel like home over the years. Good pistols.
I have large/extra-large hands and I still have to rock the Sigs in my hand just a touch to decock the pistol with my primary hand thumb. I have to do the same movement to hit the mag release definitively too. The 'flip' movement is virtually identical for both. Unless you're going to switch the mag release to the other side and activate it with your primary index finger (some do this very successfully), you're going to have to learn the 'flip' for hitting the mag release anyway. It can be very well engrained for both tasks, IME.
For smaller handed folks, we simply teach using the support side thumb, like everyone has already mentioned. It's really a non-issue.