I'm sorry to hear that your pug is going to have to have that eye enucleated. It's always hard on the owner when a pet has to have an eye removed either due to trauma or due to something like glaucoma.
After doing a little research (ie: calling one of the vets at the clinic where I work) the general concensus is that there's really no reason why they shouldn't be able to use an eye prosthesis on your dog. The only reason one may not work is if your dog's eye socket is too shallow to hold the false eye (this sometimes happens on dogs who have bulgy eyes).
I would discuss it with the vet who is going to remove the eye. They'll have a better understanding if your pup is a viable candidate for this type of prosthesis or if she'd be better off without one.
I hope this helps.
I'm not an expert on these prothesis' but I think that they generally are placed at the time of the enucleation, and once they're in...they're in.
I found a website with a link to a veterinary eye guy here: http://www.dog-health-guide.org/canineeyeremovalsurgery.html . You might want to call or email him and see if he can answer your questions.
I would, however, have the eye removed as soon as possible, just to avoid the possibility of infection (but a day or so shouldn't make much difference.)