Actually, a good hint is to KEEP (!
remember that - KEEP, not delete, Tomb!!!
;D ) several missions
in yer player folder, just dummy missions made on "Desert Island" with ONE unit (player) and then whatever stuff ya need;
I (used to..) have some missions where I lined up this & that, using the copy/paste method, and when I needed to I simply highlighted everything, left the mission, loaded my REAL missions and *CTRL+V* paste it all with one click
;D
P.S.
This is also what you should do when making the INTRO part of yer missions; just highlight the created stuff and hit CTRL+C (copy) and then paste it into the INTRO.