X-wing only. Non-Resistence only.
Per the rules that doesn't actually work. Upgrades are assigned to the Rebel, Imperial or Scum and Villainy primary factions. There is per the rules no such thing as a Rebel Alliance or First Order upgrade.
This of course could be changed, but it's fair to ask if they would just so they can keep the S-Foil card off of T-70's.
Thing is, if the point cost is low enough, say 0-1 point, and goes in the modification slot, you could make something that is useful, yet balanced for all ships with s-foils. Because it has to compete with other good modifications. It would have to be fairly decent even at 0 points to push Integrated Astromech out of that slot.