Mine never pooped on the potty chair, only peed on it. He didn't poop on toilet until the week he decided to fully train, and by then he was mainly using the actual toilet all the time. He did pee in the potty chair a lot during training, though, which we rinsed and wiped out with disinfectant wipes as needed.
We never used popper shirts under clothes. Seems like just more clothes to wash to me...They are plenty warm in the clothing they have on, so I guess I just have never seen the point.