If you want just the quality, prepare an stool score (google for bristol stool chart or similar). You can distinguish between loose and firm stool etc. But honestly, what shall be the information from that? You should think on measurements of the SCFA, pH, lactic acid, ammonia. The main question is, what do you expect providing the different potential probiotics.
I'm sorry, I didn't include the other things I planned to do such as the microbial community profiling. I hoped that assessing stool quality would tell me how each probiotic treatment affected the stool consistency compared to antibiotics only group or how near it would be to the no treatment group after a set period.