
Do NOT use a viv heat pad without a thermostat on it - I keep snakes and one of the top causes of injuries in snakes is uncontrolled heat mats and puppies will be no different when they are very small, in not being fully aware or mobile enough to move off a too hot pad quickly enough to preven themselves being hurt.
The temperatures these things can reach when not controlled with a stat is shocking, and a fire hazard to boot.
With a thermostat... i would think with care (so that one can ensure they are not chewed!) they would be ok, but certainly not without one.
I have four running at the moment, when i tested them all... 3 of them stayed around 45 to55c. one of them kept on rising.. and rising... at 90c i wimped out and turned it off!