You're right, but that's an entirely different couple located in eastern Québec. There was indeed another Jean Poulin who married another Marie Gagné (daughter of Alexis) and that couple also had a daughter named Madeleine sometime after 1726. What became of her, I don't know.
The Poulin-Gagné couple I was referring to in my earlier posting lived in the Charlevoix region not far from Québec City. Marie Gagné was daughter of a Louis Gagné and Jean Poulin was the son of yet another Jean Poulin. The two Marie Gagné were distantly related. This Charlevoix couple married earlier, in 1702. They presumably had a daughter named Madeleine abt. 1705 who married François-Xavier Thibault in abt. 1724.
Part of the confusion arising is because the family names Poulin and Gagné were already widespread by the late 1600's and the given names Jean, Marie and Madeleine were very common, so it's not improbable that two distinct Jean Poulins married two distinct Marie Gagnés and both named their daughters Madeleine, albeit with about 20 years between them.
Since my original posting, I got an answer to my query from someone who had in hand a contract that positively identified the parentage of my Charlevoix Madeleine. I've posted details about this on my website, where I discuss this Rimouski couple as well, at http://members.rogers.com/pgpotvin
Thanks for your reply. Cheers.