Thanks for the answers to my questions and the additional information.
First, let me reassure you that there's no contraindications to giving yogurt or other dairy products with Metronidazole. So, if this is the only way you can get him to take his medication, then that's fine.
Given the timeline for the development and subsequent resolution of Rocki's diarrhea and now Cassin's issues with his stools, I'd have to suspect that you may be dealing with a bacterial or viral infection....although I wouldn't entirely rule out intestinal parasites either since they can be contagious between dogs (not usually in such a short time frame though).
If that's the case, then it may take 5-7 days for Cassin's stools to become normal. But since he's otherwise acting fine, then I might worry too much.
His dose of Metronidazole is exactly what I would have prescribed for a dog this size so I'm glad your vet and I are on the same page about this medication (many vets tend to dose it really low).
I do have a few suggestions which may be of help.
1. A bland diet of boiled, skinless, boneless chicken breast and 20 minute white rice will be better for him than his regular kibble and rice together. I'd feed several, smaller meals throughout the day.
We typically recommend avoiding fat when dogs have looser stools.
When his stools firm up, then slowly transition over to his regular kibble; this process will take about 7 days.
2. Start him on a canine probiotic such as Forti Flora which may be available at local pet or grain stores. Yogurt is not a bad probiotic for humans but isn't a very effective one for dogs. Culturelle is one exception, though, and is one which I have advised in the past for my patients.
I hope this helps, especially to reassure you about the yogurt you were giving him. Deb