I would hold off on any classes for your puppy till she was fully vaccinated. I have no clue why the trainer would suggest that 8 to 10 weeks was a good age to start training , puppies are not finished with their shots at that point
You can get some training in yourself in the safety of your own home to give her a head start. Go to your local Library and see what books they have on puppy training and go from their.
Now about other dogs around your Sadie...When I first was getting my Jeter, I did ask my vet about him going around my moms Yorkies only cause I did not want to leave Jeter alone at such a young age at Christmas time. My vet said that as long as I knew that they were up to date on shots then I could let him go around them. So Iwas very confortable taking them over to my moms. My vet also said that going to a pet store, puppy park, or the vet office is not a good place to let a unvaccinated pup walk around because you have NO clue what the other animals might of be exposing your pup to. I still keep my Yorkies in the puppy purse when I go to the vets office, cause I do not know what other peoples pets are their for so I keep mine safe away from them
Best of luck to you with little Sadie.