The collars are adjustable so the size small will easily fit your Chihuahuas. The large one -20 pounds - is on the border line so we recommend you buy the large size to be safe.

This collar should fit a yorkie poo normally but 20 lbs can be little tight. We recommend buy a large size. It is adjustable so the large one will fit for sure. - Seresto collars comes 2 sizes only -

Seresto has been extensively tested for its safety, including pets chewing on the collar or ingesting the entire collar. As the vast majority of active ingredients are bound in the collar matrix and not on the surface, the only side effect recorded after ingesting a full collar was a mild stomach upset (e.g. loose stools), which was resolved within 24 hours.

You can use Seresto safely on puppies 7 weeks of age.