No tape or foam? Not even Styrofoam? hmm, I suppose it's okay if your head is built sturdily and safely enough. As for clay, if your teeth/tongue aren't HULKINGLY HUMONGOUS, then you're probably fine.
I'm hardly finished with my first fursuit, and I decided to be brave and go all-out foam style (probably not the best idea for a beginner) but I think as long as its wearable and safe, you can use any method! No matter what method you use, it's almost guaranteed that you'll be making awesome suits in several days/weeks/months/years/eras/etc!