Usually just need a tank (PLD, NIN or WAR/MNK), two melee DD's (WAR, SAM, DRG, DRK, MNK, etc.), a healer (WHM, RDM, SMN), and two other mage DD's (RDM, BLM, SMN).

That's pretty much it at those levels. Later on you pretty much need a BRD or RDM for refresh -- unless you're in a burn party (i.e. All blms, all mnk's, all melee, etc)

Edit: I lie, you need a refresher in manaburn, and probably a brd in melee (don't know too much about burn parties).