That's a good place to step in.
The actual transition between ground fight and space fight will be handled in an ad hoc (no, not you Mike) fashion, depending on the specific nature of HOW and WHEN units take off and enter the space map. However, all players in attendance should be prepared to be given at least SOME aerospace unit(s). This is why that rules PDF I put upthread exists. ASFs in space, particularly, are easy to run; if you can run a hovertank you can run an ASF.
I personally am specifically NOT going to be assigned a "side" until we have a 100% confirmation of everyone else's attendance AND what force they'll be playing for. I am aware that I am the resident aero ace, and that my presence on either side - if not carefully balanced with other players' skill - can make this go from a difficult scenario to a stomping. Doubly so because I expect the Clan players - myself included, if necessary - to be absolutely playing at the top of their out-of-character abilities.
My very strong suggestion to the players of the scenario is to create a plan in some significant detail, and understand the concept of "acceptable casualties". While the scenario is winnable, if you are determined to board and/or recover *every* unit on the field, then it will likely not be winnable after all. You should consider which deployed units of both the GDL and Rangers are acceptable casualties if they cannot reasonably make it back to the DropShips.
Gray Death players - on the day of the scenario - will be given a packet regarding their tactics. It will not be irreconcilable with your battle plan, but they will have their own priorities as well (no, they aren't going to form a shield and all valiantly sacrifice themselves so the Rangers get away scot-free).