It's good practice to alternate between 12-16 different soldiers so you always have trained professionals ready to go. However always make one soldier (preferably a support) someone you drag with you on every mission possible to promote them quickly. Reason for this is to unlock all the Officer Training School abilities sooner.

As for the terror mission? I'd take it immediately simply not to risk it timing out before your soldiers recover.
Just play it smart and cautious with your squad, keep them close, piss of minimal enemies at the time, make good use of overwatch, and remember your priority is to keep your men alive, not to run around like maniacs saving the civilians despite what the game tells you. If your squad dies the mission fails, but once you win the mission any citizens still out there who weren't killed still count as being saved.