Hello, well of course every Yorkie is going to be different, but I'll tell you what I do.
What do we feed them? I feed Jackson Acana grain-free line right now. We rotate (meaning when one bag is done, we switch) between the Grasslands flavor and the Prairie Harvest flavor. It works awesome for us! Two small firm poopies a day that are easy to pick up from the yard and don't even smell
that bad (for poop, anyways
). Other foods I like and recommend are Fromm, Nature's Variety, and a few others.
What shampoo and conditioner to use? That is an ongoing process for us... lol. I'm still looking for that perfect shampoo/conditioner where the smell lasts more than 2-3 days. I like Minksheen but some people don't like the cologne-ish smell that is has but it makes Jackson's hair sooo soft and shiny and he smells really good. A lot of others use Yorkie Splash and Shine (made by YT member WalnutHill), Nootie, BioGroom and Earthbath (the last 2 can be purchased at Petsmarts, etc).
What tooth paste to use? I just bought some doggy toothpaste at Petsmart. I really don't know the brand name honestly. I also purchased a spray bottle of Petzlife to maintain his teeth. I usually spray his teeth (2 sprays each time, one on each side of mouth) once a week.
Is twice a day normal? I don't think more than once a day is necessary but it wouldn't hurt to brush more. Jackson's got pretty good teeth, he's still young at 1.5yrs old, and I just recently began brushing his teeth. I *should* brush more but I usually end up brushing his teeth 1-2x per week and it's been working for him. I figure it's better than nothing.
When is grooming to much grooming? I'm not sure what you mean by this... but I take Jackson to a professional groomer about every 8-10 weeks depending. I keep him cut fairly short though.