As for puppies all of mine that were trained as puppies trained in places with people and dogs everywhere. Both my older golden and female GSD were in the middle if a down stay when another dog broke their stay and were jumping all over my dogs, my dogs didn't budge. I think that all the distractions up to that point helped them with their training.
Misty- Samoyed Mix, Tannor- Golden Retriever CGC
Robyn- GSD CGC, TC, Midnite-GSD CGC,TC, Brennan-Golden Retriever CGC, Batman-Husky/Greyhound , Apollo-GSD