Regens stacking isn't a problem because you can't cheese mechanics with stacked regens. They're very hesitant to allow stacked GCD shields because you could cheese mechanics with those. They haven't come up with a solution for the cheesing problem, so GCD shields don't generally stack. If you want Noct shields and Galvanize to stack, you have to do something about being able to survive mechanics by cheating your health pool up.