Does the Quebeckism "pis" mean exactly the same thing as "et" in France (or in formal usage in Quebec), or is there some nuance between the two?
Quebecer here, and I can't think of a significant one except that it can be found in a use similar to "so" before greeting: "Pis, ça va?" ("So, how's it going?"), and "et" alone is found in numbers (*vingt pis un does not occur).
You may encounter "et pis", though, which is kind of another variation (and will also be the informal pronunciation for "et puis" meaning "besides").
In fact, as far as I an tell, the entry for "pis" in my Dictionnaire Québécois D'aujourd'hui is a reprisal of the entries for "et" and "puis" (I have only used the differences in the uses that correspond to "et", but "pis" also stands for "puis") with some fixed expressions and the examples as the only differences...
Et is and and puis means then (pronounced like pee in Quebec but it is often mistaken with pis like le pis, (the worst). It's the local accent and dialect in Quebec that makes it confusing for everyone outside of Quebec and a small population in Eastern Canada.