@boxcar Even if it may feel sore, I don't think your butt could be badly hurt by a rough stool assuming a reasonable diet.
FWIW, for around a decade or more, before I came into the heyday of anal play, I had a persistent fissure that really crimped my capacity to take what was my first toy, a reddish jelly buttplug. I think there may have been some fungus, and the way the fungus of athlete's foot leaves you raw, I was sore right in the ass-crease. (The dermatologist who looked at it said it wasn't fungus.) Whatever it was, I eventually solved it with an herbal salve and time and good luck. Since then I've amazed myself 😃
Regarding overstretching, I think most people are capable safely exploring their limits. I know I have to psyche myself when going to the next level, develop my hunger, imagine taking it inside. Sometimes alternating with an incrementally smaller toy that I can take successfully. Just like the vast majority of humans can rely on thirst to regulate their fluids, I think most can rely on pain to bring a halt when play exceeds one's capacity.
As an aside, I'll observe that one develops humility when trying and failing to fully insert a toy. It leaves me trembling, with a gnawing hunger to take it in some future session, scheming how to train and prepare myself.