that's crazy enough alright. But if you aim to use all the planes on battle against AI, you could unlock them manually by editing the plane's .ini file and just downloading a battle scenario. There you could use any plane you have available.
For the campaign part i would not really recommend it since its kinda broken and its too much effort for just like 13 missions

you just gave me an idea.
I will bump the plane count on the campaign shop to 15 as well as change the order and price of things following the exemple of Ace Combat zero(Cheap plane has low cost weapons, but all weapons are usually 20%-30% of the plane cost)

if you wish, i could send you a copy of my modified file, or just the template for the scrip to add new planes, and you send me back filled with another couple of planes, prices and everything, maybe, someone will see this and wish to add a mission or two, how about that ?

Update:
I have included 15 planes into the campaign, most of them are playable and i also changed the starting planes, all playable planes will use all vanilla skins available in a game's fresh install, but i ran into some little problems.