I love this handy chart from Innospec’s presentation: “Formulating High Performance Sulfate-Free Cleansing Products”.
As you can see, surfactants have a working pH range. Some surfactants can hydrolyze at a low pH (SCI, Iselux, Sulfosuccinate).
As most surfactants are diluted, we have a handy calculator which does the math for you so you know how much to add of each surfactant: http://makingskincare.com/surfactant-calculator/
Which sulfate-free surfactants do you like to formulate with?