I agree that some yorkies might be very UNsuited to be trained to help you in practical ways, and some might be great at it!
My nine month old is the most endearing little lovebug, but not terribly bright, and I can't see her being very helpful!

My puppy...hard to tell with her...she is still in that wiiiiiiild puppy stage!
But I think that it is VERY likely that you could find an older yorkie that would suit your needs brilliantly! As stated, you'd have to be careful where/who you got it from, as some people will tell you ANYthing to sell a dog

, but yeah, rescues, shelters, or trustworthy breeders would have a good peg on the dog's strength's/weaknesses.
GOOD LUCK, and keep us updated!!!