I just had an idea as to how to help if you do take your Yorkie (or any dog) to petco or petsmart for grooming. Write down a list of what you want and make 2 copies. Give the list to the groomer and tell them this is exactly what you want (you might include on the bottom a statement that says you won't pay if it is something completely not right (like you said trim and the dog comes back shaved) and ask them to sign the list you keep so when you go back and it's not right you can show it to the manager. You then have a contract with them in a sense.
I've never had Sophie groomed there so I don't know if you have to sign anything or not, but if you do see if it has something similar to a no-pay clause in it or something that says you have to pay regardless. The make your own contract is sort of skirting contract law, but it might work. Can you tell I worked for an attorney for 6 years (dealing with contracts) and I'm married to one, too?