So what is the possibility that people with the same surname in Poland are related in some way(Either through bloodline or a relative by marriage)?
I've met many people in the U.S who bear the same surname, but have no relations.
Take for instance the name 'Kowalski' which means 'from the smith'. Considering the number of smiths there were in the Middle Ages and later, I think the answer is clear that somebody in the center of the country could have been called 'Kowalski' as somebody near the borders could have been called 'Kowalski'.
Are they then related? No they aren't.
What about the less common surnames? Kurowie, Kurek, Kurski, Kurowski?
That's the same as asking if people with the name 'Swiatopelk-Czetwertynski' (a Belgian princely family with Polish roots) are all related. Well, because there are so few people with this name, it's very likely they are related.
I think it's reasonable to assume that the larger the number of people with the same surname is, the smaller the chance is they are all related.