Missions written for FS2_Open will only play on open (especially if they use the new SEXPs like End Mission). Most of the community has moved over to open anyway though so you wouldn't be cutting your audience that drastically if you did too. Pretty much the only ones who haven't moved are those who can't run FS2 Open rather than those who won't.
Using a red-alert will give you the short 1 page briefing you saw in the main campaign but you can always red-alert and then simply not make the following mission a red-alert mission (giving you a full briefing).
Beware of the end-campaign SEXP. It doesn't do what you think it does
In the campaign editor there is a SEXP called End Campaign which causes a message to pop up saying that the campaign is over when you try to enter the ready room.
The End-Campaign SEXP in the events editor however is a completely different beast. It only works when the campaign file is freespace2.fc2 (i.e the main campaign) and although it does end the mission it also triggers the end of campaign cutscenes.