I noted in the instructional video that they recommend using aged polymer clay. If you need to not wait a month or more to start your sculpt, you can leach the clay. To do this roll it out using the pasta machine and place each sheet between two pieces of typing paper. Take these paper clay 'sandwiches' and roll them up together and then sit on it for about half an hour. This will make the clay more suitable. If it is still too soft just repeat the process with new typing paper until it reaches the desired consistency. Roll it all through the machine again to get the plasticizer more evenly distributed and you have a nicely usable sheet of clay.
I tend to like a really firm clay when I am doing my usual sculpts. This is the first time making a baby head and therefore the first time using a secrist armature. I found pretty quickly that Mr. Secrist's idea of firm and soft clay are vastly different from mine. From the head sculpting video I would have to say the stuff they said was soft was somewhere in the vicinity of syrup for me and the stuff they said was firm was still way softer than how I normally go. I found out really quick that a sheet of clay at my normal stiffness simply would not wrap around the form without cracking.
I was at the factory being certified as a Newborning Teacher before the first sculpting dvd came out. Vicki Reid was there too and she showed me how to use the sculpting form and pasta maker for the clay (amongst some other sculpting tips!) and the clay she was using was quite soft. So I know what you mean about interpreting the softness/hardness of clay being an individual thing. It was too soft for me. I usually pick a medium softness and blend it with my older aged clays. Then it's just about right for me. :)