he just meant that the infinity has a lower xmax, which means that it will hit its limit first.
with 500 watts on two drivers in a pb12 clone, the infinity drivers will be just touching xmax, so the extra excursion of the alpine's may not be a factor.
that said, the infinity drivers would be happier in a larger enclosure. if you could go with something closer to 8 cubic feet, the frequency response would be flatter.
if you can put a little eq on them, with 500 watts either driver will work.