Probably a crazy question but how does one know when it is ok to mix products? For example, say you want to use Skincetuicals CE Ferulic in the morning and a Skinmedica Renova product at night? When not sticking to one company how do you know what products should not be used together? I am trying to add some things to my routine and just wondering if anything is going to react against each other. After months of a very basic routine (nothing more really than a cleanser and moisturizer) and feeling that my skin is finally "calm" I feel that I can now add something more.