Absolutely you can overdose with ACV. I dare you to sit down and drink a full 8 oz of undiluted ACV...tell me what you think after that!A sour crop is already too acidic. Adding more acid isn't going to help. A probiotic might help, such as yogurt. But ACV is not a probiotic. The unpasteurized stuff may have some live enzymes and yes, live acetobacter, but acteobacter is not considered a probiotic.
A tablespoon maybe.