In universe, they totally could have used Y-Wings in that movie and it would have made sense. Those things are like the Ford F-150s of the star fighter world. They've been around forever (since the Clone Wars) and everyone and their mother knows someone that has some. All you have to do is get onto Jedi Craigslist, type in "Y-Wing" and bam! Page after page of cheap, easily obtainable star fighters. That said, some questions on there don't really need answering, such as the clone army. Other questions though, such as who the Resistance are and their affiliation with the New Republic could have easily been clarified in the movie within a few minutes and a few sentences. I kind of chalk that up to sloppy writing. Shouting out to you PlaceboEffect so you're not left out of this loop.