What role does the NPC party serve? What is their agenda?
In the campaign I am building I have (so far) written up three ally/rival/enemy adventuring groups that are active in the region the PCs will begin adventuring in.
The first is a group is from the Theocracy of the Pale (Greyhawk). They are devout followers of the deity Pholtus and seek to evangelize to the people of the Shield Lands. Amongst their ranks is a Cleric/Rogue/Shadowbane Stalker, a Paladin/Rogue/Shadowbane Inquisitor, a Diviner/Master Specialist, and a dozen Pilgrims (commoner 1).
The second group is a collection of dwarves who are preparing to explore White Plume Mountain and the surrounding area. Amongst their ranks are three Fighter/Clerics, a Bard, and a Rogue. All low(ish) level.
The third group is a preliminary scouting group from the Empire of Iuz. They are lead by an unamed member of Iuz's Lesser Boneheart. (I haven't decided which.) This group contains three Wizards of undetermined power and a Cleric. I am still playing around with the classes/prestige classes of this party.
In your case, I am assuming that the NPC party is supposed to be a well-rounded adventuring party? Possbily serving as rivals or enamies to the PCs? I think a good rule of thumb for five NPCs in combat situations is to have two characters who serve as the primary melee combatants, one who can buff the party, one who can control the battlefield, and one who serves as the primary ranged combatant. Out of combat, you need someone who can heal, someone who can be the party face, and someone who can scout/gather information.
There are a few ways you can go. The first is a higher-powered route with a Bard, Druid, Cleric, Cleric, Wizard. The Bard serves as the party buffer in combat and can serve any of the out of combat roles. The Druid can serve as the primary melee combatant as well as a healer, face, or scout out of combat. The Clerics can be built to have one fight in melee and the other at range as well as being healers and possibly faces out of combat. Finally, the Wizard can serve to control the battlefield in combat while using divinations to gather information.
A more traditional approach would be Cleric, Fighter, Paladin, Rogue, and Wizard. They would also cover all the roles though not as well as the other party.
Unless the party has a specific agenda (like the ones I list for my campaign, and even then I was careful to consider the IC/OOC roles) I think that as long as the NPC party has someone to serve the roles I listed above any character classes should be okay.
One interesting thought would be Barbarian, Ranger, Scout, Spirit Shaman, Warlock. It would be on the weaker end of parties but still cover the roles. This party would work well in wilderness settings. Possibly even originating from a tribal area. A Dragon Shaman could also fit in here.
Another thought could be a holy army of Clerics, Crusaders, and Favored Souls. You would loose a little of the battlefield control aspect here but more than make up for it in other areas.
