Personally I use the "birthplace" method with limited planeset as appropriate.
A fixed set of planes should also be doable, just check the "idle" checkbox on them so AI don't fly away.
In my opinion the new "coop approach" is to let the e-pilots join in cockpit and chit-chat during engine warm up and as such I tend to use a trigger just outside the airbase to "start the mission".
Alos possible is to use the ingame "Mission" menu (TAB 4) but that requires a good amount of scripting (is available in my "OdenCoop" shared over at simHQ if someone want to have a look at the code, feel free to copy-paste).
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