Q: Do you have to use an entire line of products for them to be effective?
A: Short answer: No. Though certain ingredients do work synergistically (meaning that they boost each other's effectiveness, and using them together creates a greater total benefit), most provide only an additive effect, says David J. Leffell, MD, professor of dermatology and surgery at Yale School of Medicine. Two products containing a variety of antioxidants, for example, give you the benefit of each antioxidant, but nothing more. So it's fine to mix and match as you please—unless you're using a serum, say, with alpha or beta hydroxy acids and a moisturizer with retinols, because the combination of those ingredients could result in an exacerbation of potential side effects, including flaking and irritation. Be sure to read labels very carefully.
Bottom line: Use one or use them all; there's no magic in a line of products that makes them work better together than alone.