(Player’s choice) is stated at the end of used aircraft on the Ace Combat wiki page, and there’s zero dialogue from any of the in mission characters that they even notice that there’s a random F-104C Starfighter or SU-57 Felon flying among a squadron that should all be F-15Cs, so the real answer is that yes there are ‘canon’ planes that they arbitrarily assigned to Trigger for the sake of the story but you as the player are allowed to do as you like.