I don't think it's a good idea. Besides what Jane said, there are enough unwanted mixed breed dogs (and purebred dogs) to be creating more. Also, if I'm not mistaken, pugs grow bigger than chihuahuas, right? That means that the babies will be about a mix of the 2 in size (maybe... not as big as a pug, but not as small as a chi). This would mean that the babies will not be small enough to pass through the mom's tract. This could lead to the need of a c-section, death of the puppies inside the mom and even death of the mom.
As an anecdote... where I work, once a family arrived with their dog. A dashund, she had given birth to 3 puppies and 2 days later they found a fourth dead one. The mom was depressed and they took her to the vet. When radiographs were done (for the first time in this dog) the Dr. found 3 more dead babies in her. He performed an emergency c-section, the dead pups stunk the whole clinic... they had been dead in there for a while... The dog stayed hospitalized, but already had a septisemia (blood infection due to the pups decomposing inside of her) and died. The family payed over $2,000 and their dog died. I don't know what happened to the pups, but raising orphan puppies is not easy at all, specially this young.