Hello,
Check your messages, I inquired about the doses of the 2 medications & the concentrations or strengths of them. They are harsh on his system.
The soya yogurt is fine. That is a probiotic & while some vets don't recommend them, a lot of vets do. I have personally seen much better effects using probiotics than without. Those medications that are particularly harsh on their system can contribute to increased bad bacteria killing off the good bacteria which can encourage yeast & fungal infections.
The soya yogurt is dairy free, so you can use any flavor that you want. It has live bacteria in it which is the same thing as a probiotic only in a yogurt.
As suggested, you can give .5ml daily, that should be adequate for your sized dragon. I would give during the entire medication round as well as up to 1-2 week post medication. However, let's review your doses of the Septrin & Flagyl first, because given together that can be dangerous.
Be sure to give extra oral fluids as well, during the medication round.
Tracie