Realistically, even if the Imperials have lost their edge from the post-Clone War era, some of their formations have seen extensive combat against formidable foes, but the whole Imperial Navy benefits from a high degree of training and a high and consistent standard of training for their TIE pilots. The rebels have a long way to go on catching up with them from the standpoint of organization. No one's saying they're starting from scratch, but merely the obvious; they are at a disadvantage in terms of experience as well as organization. The empire and the imperial military are established. The Rebellion is not so much. Also, while it's fair to say that the average Imperial army conscript is probably not well motivated, TIE fighter pilots form an elite within the system and are professionals. The rebels may have a lot of raw talent, but they have yet to really translate that into results as of 0BBY. Bear in mind, this is before Luke Skywalker downs the Death Star.

Equipment wise, it's a different deal. But half the RP will be about teething with the new gear. Sure, the Rebels have experience with Y-wings and Z-95's, maybe even ARC-170's, but the X-wing is a different platform, even if it is similar. Also, even the characters with mercenary experience are going to have to learn more formal fighter tactics as well as life on a starfighter carrier. Incidentally, I chose the class of ship I did because it's more carrierlike than even the Imperial Star Destroyers.