Dear Alexander,
You not only have to be concerned if the active ingredients will be inactivated, you also need to be concerned if there will be any reactions among the ingredients that could result in adverse effects. For instance, if some of the products are acidic and some are basic then there will be an acid-base reaction that can result in the generation of gases. In a closed container this can build up pressure and release explosively when you open the container. The cream that is formulated with benzoyl peroxide uses ingredients that are compatible with the peroxide which, chemically speaking, is an oxidizing agent (a bleach). These are notoriously reactive with many other chemicals - mixing it with other formulations would not be prudent.
Without seeing ALL the ingredients in these formulations you want to mix no one can give you a definite answer. And considering the number of permutations of reactions that could take place this is just too complex to predict.
You have to consider that, if it was really possible to mix all these different ingredients to get all their different effects in one product, a professional formulator would have done it. There are probably fundamental chemical incompatibilities that do not make that possible.
In general it is a bad idea to mix commercial formulations, especially when they contain acids, bases, oxidizers, biocides, etc. It is a better strategy to examine whether you really need all these different products or if some of them are redundant. For instance, glycolic acid and lactic acid fulfill the same purpose in a formulation - they are both alpha hydroxy acids that exfoliate the skin. If you are using two products one after the other where one contains glycolic acid and the other lactic acid you're exfoliating twice - you should consider whether you need that level of wear and tear on your skin.
Karen
