
I've had much smaller pups and always 100% start worming at 14 days. Then at 5 weeks, again at 8 weeks, and again at 11 weeks. You can't let them go until 12 weeks so it's no big deal.
I have used Panacur 10% for donkeys years with no problems at all. My bitches are wormed during pregnancy too. Common sense tells you that your pups are already used to Panacur before they are born. Mum is wormed along with pups once born, but she is dosed at the normal adult dose
for one day only, not 3 days like her pups.
In all my years with Chihuahuas I have never had a problem using the above regime. I did used to wonder at the dosing years ago, but I now accept that it's possible to have a dose as small as 0.02ml!

As far as heat is concerned, at this time of year I have the heat pad going for 6 weeks solid
minimum. I do have one bitch that likes it on for as long as she can. Even in summer!