First of all, how do you know what products you can safely layer?

For example, if I want to use the La Rache Posey Active C, do I put it on and then my moisturizer on top of it? I know I can layer the Renova.

What about if you were buying a new product...like the PCA SKIN pHaze 24 Rejuvenating Serum, for example? I don't want to create issues here.