I don't think the demons had any specific body temperature. Like the others said, just do whatever suits your story best.
Also, I think having Lucifer burn "cold" was a nod to the Divine Comedy. At least that was my theory anyway. I'm not sure if it was ever mentioned that it was an indication of the vessel's life state, but that's a very cool theory.
I would say base the body temp off of whether or not the vessel is still alive or not. Lucifer's vessel was basically dead without him, same as Crowley, but the people temporarily possessed by demons were still alive so they would be warm. If the vessel is dead, it would be cold. :)
Also, I think having Lucifer burn "cold" was a nod to the Divine Comedy. At least that was my theory anyway. I'm not sure if it was ever mentioned that it was an indication of the vessel's life state, but that's a very cool theory.
And mm, yes, Hook.