If you want more survivability as a Mage, go Mage/Priest or Mage/Knight if you're human, and Mage/Druid if you're an elf. The ability to heal yourself can help negate squishiness, and anyone with /Knight will have higher defense. Other than that, Mage is one of the most balanced classes in that no secondary combo stands clearly above the others. I know high-level Mages with every secondary possible, and they are all good.
You can have three classes, but only two are active at any given time. So for example, if you were to choose Rogue and Druid as your second and third classes, you could play as six different combinations: M/R, M/D, R/M, R/D, D/M, and D/R.