[Ellie May]

I like Interceptor (Sentinel without the chitin inhibitor for fleas) for Yorkies.
Sentinel is also fine if you want the added Program.
heartgard is okay too.

You can use Interceptor when they are 2 pounds and he should be by 6 months. You are seeing the 6 month thing because it takes that long for the larva to go to the heart and turn into adults. So basically a pup less than 6 months old shouldn't have a problem with heartworms at all. I would be on the safe side and start probably around 4-5 months if possible.