Aww!! Don't be upset!! I think I had the same exact experience as you.. I took Duncan to get groomed at PetSmart once, and NEVER again.. I don't know, I just didn't like the feeling I got from the people who worked in my specific PetSmarts grooming dept
(although I'm sure it's different everywhere).. I couldn't wait to go pick him up from there..
I could be wrong, but I think it's like really really obvi if their nails get cut too low.. I've heard they bleed like crazy (at least that happened with my cat) but I don't know..
AND I also had a trainer tell me they thought Duncan was part chihuahua, I was sooo upset as well! It's like I had my own identity crisis on his behalf... lol... Then I realized that he was soo young at the time (like your puppy is) and all yorkies are different.. which is what makes them so great in my opinion
Everyone here is right, they aren't specialists on every breed there so I wouldn't take what the trainer said to heart