Kurt "answered" my question about the flatten legs and feet in the subdivision! :attn:

After some time, in the cartoon's dog video tutorial (part 4), he says: "(...) hit 3 (...)" - when he is adding more detail and he converts the model to subdiv again. So I pressed '3' in the number pad, in my keyboard, and the flatten legs and feet got smooth!

Anyway, thanks for your attention!
Sepraud