Multiplayer guidance...
From experience, it is found that in Multiplayer flights it is preferable that in general, everyone uses the default aircraft and default scenery for the flights.
...otherwise...
....downloaded aircraft and downloaded scenery will not show up on other player's monitors....trust me...!
The only other way is for everyone in the fight to agree to use pre-selected downloaded aircraft, and pre-selected downloaded scenery well in advance of the flight...!
Like it, or not, it is one of the peculiarities of Microsoft Multiplayer flights...!
Cheers all...!
Paul...G-BPLF...!