Well, another thing to consider is that the early Alliance fleet was made up of various types of ships, not all of them designated as true warships, from all the different rebel sects. The only proper warships that are in the Alliance fleet that you tend to see a lot of are the Nebulon-B Frigates and Corellian Corvettes (which is debatable since they weren't exclusively fitted for combat only roles). The Mon Calamari vessels are still technically passenger liners that were modified and even then, didn't see any major use until after The Battle of Hoth (honestly though, there are so many conflicts as to when Calamari Cruisers began to be employed by the Alliance Fleet. I'd like to think that the Rebels had a few of these in their arsenal from captains and crews that had been able to escape the enslavement of their homeworld, not to mention Ackbar was rescued before The Battle of Yavin and had earned the rank of Commander shortly after his rescue).
Of course, one thing that needs to be understood, that if a pre-Yavin campaign were to be made, we'd have to go and add ships that were being used at the time, mainly a new slew of TIE-craft (namely, the TIE-gt, TIE-fc, TIE-rc, TIE Vanguard, and TIE Scout) as TIE Bombers were just entering service as a replacement for the TIE-gt and TIE Interceptors weren't around).
Also, craft should be prioritized as a need basis as for what the story calls for. Since we're kind of doing the opposite right now, we're limited in terms of what craft we've decided to focus on.
With regards to the Y-wing, I think heavy fighter/light bomber is an appropriate role for that craft. It should be able to take down small heavily armored targets such as transports and freighters, and at least some of the older corvette-class vessels (well cause damage anyways).
