I'm sorry I don't have a link because it is just something that I've heard of TV many times before whenever this kind of topic makes it on TSN or something. But... The minimum arena capacity for the NHL is (or at least was) 15,000. When the new Winnipeg arena was being built they wanted to make it with the minimum standards for the NHL, just in case. Perhaps someone in the Peg can confirm this. And I also remember hearing that it was easily expandable.
So Quebec would be okay capacity-wise. Not that I think it matters, just trying to clear that up.
There is no minimum capacity for NHL arenas, and True North (the company who built the building) did check with the NHL if 15,000 seats would preclude them from future consideration. The NHL did not, and does not, have a problem with the size.
As far as expansion goes, there is the possibility of adding 12 additional suites below the current row of press boxes, as well as the possibility of adding one more extra row at the top of the rink. This would add about 500-600 more seats and could easily be done in the offseason.
Beyond that, though, it is not easily expandable. Doing so would require raising the roof and probably annexing at least one of the surrounding downtown streets.