On all my playthroughs, I've sent Grunt back with the crew, and Mordin has always survived - although thus far my playthroughs have been with soldiers and I have normally used biotic/tech squadmates.

I think the most important thing is to try and make sure the squad has a few characters with some combat skill. Grunt and Zaeed are the combat specialists, and Garrus has experience in "holding the line" (when you find him his been holding position for quite some time!) These three characters will basically guarantee the squad survives, although I have done it without either Grunt or Zaeed.

If all your characters are loyal, and you have had no-one die up until that point, as long as you have either Garrus, Zaeed, or Grunt in the behind team, everyone should live.

Btw, the lowest survival ratings for holding the line are Samara, Jack, Tali and Mordin. Afroninja's squad survived because he had Zaeed. Legion would have helped, without Legion, Mordin would probably have died, but without Zaeed, the whole squad would have died.