I know some of you add supplements to your yorkies' food. I would like to know what these supplements are and how they help. 
 
Lulu is 5 months old. She is currently eating a mix of Royal Canin for puppies and Wellness for puppies. We tried moving her from RC to Wellness only, but she seems to prefer Royal Canin. I don't know if it's the taste, or the smaller size that she likes more. We also tried mixing in Pedigree, but her stool was much softer and stinkier 

 (it didn't happen when we introduced wellness at first).  
I'd love to know how you use supplements, in what quantities you use it, and how you mix it in with your babies' food.  
Thanks!