Using both cook and serve pudding and instant pudding in pie
I doubt it would make the milk curdle, but it may mix with the other layers if you pour it in while still hot. I would cook the layer with cook-and-serve pudding and let it cool in the pot until it's more spreadable than pourable. It should be just about room temperature by then, and then you can layer it on top of whatever layer it's supposed to be on without any worries. If the butterscotch is the first layer, you can pour it hot into the pie shell and wait until it's cooled before adding the other two layers.