The group I run just added their 7th character. Before that there were 6 players and doing fairly well in the 5th module:
Elf Ranger
Dwarf Fighter "grappler"
Human Pal/Cleric 25%/75%
Human Cleric
Elven Rogue
Human Wizard non specialist
We try to avoid the weird classes as much as possible. The cleric did just take a level of Ruincaster and the ranger wants to take a level of Justice of weild and Woe "human haters".
I think with the ideal 6 person group you have 6 "units" of character to spend. You want:
2.5 units fighters more melee focus, but ranged too.
1.5 units Cleric
1 to 1.5 units Mage
.5 to 1 unit Rogue
The rogue is the least useful, but at a few points VERY useful. You could just make those points easier by letting the party prep with find traps spells, etc by giving them an early warning. In that case the rogue could almost be dropped entirely.
What ever you do, don't skimp too much on cleric or Mage. They make things a LOT easier through out the whole thing.