Small breeds need feeding little and often or else they can get Toy Breed Hypolgycemia, due to low blood sugar levels.
Try feeding her a complete dry puppy food, so she has to chew it and should eat it slower than the easy to swallow tinned food. You can soften it a bit with warm water and add a little tinned meat to make it tempting. Don't allow her to eat more than her share at any meal time, and feed her at least four times a day. You could make the breakfast or supper meals a milky porridge type food, such as weetabix or readybrek.
Give her free access to water, she may be drinking a lot because she did not have free access at the breeders.
Are you feeding the same food as the breeder, and did she give you a diet sheet with her? (I sure hope so, if you paid �350 for her).