Hi Ginger,
No need to argue, I think we're just coming from different perspectives and experience possibly.

I'm a licensed make-up artist in my state and a large part of our licensing program was learning about skin care, various products, how they are made and how they react on skin. We received the same initial training that licensed cosmetologists and estheticians receive in the first part of their programs. The initial training is all the same and then we break off into the specialities we're interested in (mine was Make-Up Artistry).
There are only so many ways a cleanser, moisturizer, toner, etc can be made and only so many ways to combine the ingredients that go into them. When lines start specializing in different products for oily vs. dry vs. combination skin, they start changing the types and proportions of ingredients used. These combo's/formulas are generally proprietary to their line, and if one were to mix those lines to handle a specific skin problem (oiliness, dryness, t-zone, etc) the combination of products may not work as effectively as using products formulated to work together. Whether or not it will cause "warfare" on any particular face is a function of the user's sensitivity and the relative difference found in products across lines.
That is what I was taught by a licensed professional and it has been consistent with my experience in and out of Mary Kay.
What I ultimately took away from my training was that it doesn't matter that much WHAT you wash your face with, just that you do. Same for moisturizing. That is what I tell my customers. I despise scripts and I'm very much the rebel when it comes to shooting down MK-ism's that get floated around.
And yep, I've known about (and read) Paula's book(s) for years. It, among many, was on our reading list in licensing class.