The dietary brand restriction was probably chosen because she is very sensitive to certain ingredients in dog food. It's very, very common in dogs .
UNless you identify the probel imgredients and feed accordingly, you'll suffer the consequences at both ends, and its not her fault.
Petsmart carry a huge range of food. You need a food that's advertised for small dogs with a sensitive stomachs; I 'd pick one based on rice , poultry, lamb. vegetables. ( Avoid wheat,and rich red meats like liver and beef, eggs and milk ) .
Feed the the same meal twice a day. Just one brand. exactly measured in a scoop. The amount is what will all be eaten up right away.. If they don't clear the plate, reduce the portionuntil you get it right . Same person feeds the dog; in the same quiet spot, and leaves the dog in peace to eat. No patting,chatting, attention or distractions. If your mornings are a mad family rush to school.nursery etc, give the dogs its breakfast in peace after they've gone. Then about 10 minutes later, a toilet walk on lead for half an hour. If no result, do the same again in half an hour.
Last meal at 5pm so it will poo before bed time )
Don't offer a new dog a gourmet variety of foods, tinned, pouched, dry, all different ingredients and flavours . You want to find one balanced food it likes eating, and digests well; and get the dog into a healthy eating habit of no more than two meals a day. Many large active dogs do very well on just one meal a day).
(Later, only when when you know it well, you can introduce yummy dinner treats like cooked carrots and brocooli stalks, cheddar cheese, cans of sardines in oil)
In between the two meals, give the dog's stomach a rest. Dog physiology is designed to for that.
Seriously limit commercial treats; preferably zero till you get your dogs digestion settled. dogs get far too many . A) treats often contain the very irritants that
upset the dog, in flavourings/colours ect).
B) the constant stream of calories means the dog won't have a proper appetite for its meal.
Once you have established a good appetite and a regular calm feeding routine, there's more hope of regular poo; and the ideal you're looking for a is a poo that's a shape and texture tou can grab with the poobag.