Thanks for writing in,
Sorry to hear your little girl is having such a rough start. Sound like alot has already been done for your girl, but here are a few other suggestions for you and maybe you want to talk to your vet about them.
The first is testing her for Giardia. This is an intestinal protozoan that causes diarrhea. It is not usually detected in a regular fecal exam so needs to be a special request to a diagnostic lab.
THe second is if it is pancreatic insufficiency where her body can't digest fats so they pass out undigested and your dog doesn't absorb them so is skinny. I wouldn't say mini sch are the most common breed affected, but it is something to consider.
This could also be a malabsorption problem and requires an intestinal biopsy to help determine what is going on.
Has your dog already been on Metronidazole? THis is my favorite medicine for diarrhea. ALso, consider asking about Fortiflora which is a probiotic and helps replace the normal bacteria in the GI tract that the antibiotics could be killing.
GIve those things a try and let me know if you have any other questions.