Clerics are incredibly good. Animte Dead as a 3rd level spell is gamebreakingly good almost. Mages are MUCH better in 3rd vs 2nd ed, as they get bonus feats. While I wouldn't dare to run 2 full progression wizards, 1 is a very good idea in IWD2, as they'll cast very, very strong spells with Greater Spell Focus, and Greater Spell Penetration. That, and they can use spells that will suck later on freely, which sorcerers in 3.0 cannot, as they are stuck with them. 3.5 let you switch the odd spell! Anyways, consider 2 Sorcerers if you like arcane (and yes, most problems can be solved by massive AoE spell damage, and or judicious use of Animate Dead), but don't run 2 Sorcerers unless you have 1 Wizard. You will often be a tad short of scrolls, but you will VERY MUCH benefit from those extra feats vs a Sorcerer. Also, 1 Wizard can cover for Concentration, Knowledge Arcana, Alchemy, probably Search (due to high Int), Spellcraft etc, they end up with plenty of actually useful skills. Well, kinda useful. But still, you really are missing out if you don't take a Wizard, those feats make him a MUCH more useful caster than a Sorcerer in many situations. Heck, you can take Sub-Vocal Casting ffs, something no Sorcerer will ever be able to without sacrificing a badly needed feat.
If you want a smaller party, I think the engine will probably end up penalizing you pretty harshly for XP after awhile, as its your ECL average iirc that rewards you. HOWEVER, a well known trick of great evil is to start with 4, then add in two characters later as lvl 1s, so they will greatly reduce your average level, and just retire them whenever they start getting too high level. Otherwise, you'll pull really bad xp after awhile, as you get to too high a level for the challenges, meaning the game will be harder, and it's already the hardest IE game imho. Be prepared to reload often due to critical hits if you play on a difficulty level that boosts damage... everyone can be one hit KOed by a crit from an arrow on Insane for several levels. Infuriating actually if you aren't pretty careful to keep healed.
IMHO, aim for a Barbarian (consider 4 levels of fighter) with maxed out strength and cons, probably wielding a two handed weapon. Aim to include a dedicated Archer, IE maxed out dexterity, probably lowish strength, relying on TWFing and Weapon Finesse for melee. Rapid shot is just dreamy early on. Strongly consider 2 clerics, but do not go without any. A Paladin is nice for a certain sidequest (...very, very, very hard fight involved with this side quest), but Paladin or Monk should NOT be spokesperson, they'll turn down rewards. Which is cool, but still sucks. You do need some Rogue abilities, but a multi-class Rogue is good enough after probably level 5ish. Early on, pure Rogue will have more luck then a multi, which might have trouble with the odd trap.
Druids are decent, Monks are decent, Rangers are not unless you dual out asap, but watch out for XP penalty for unfavoured class! Bards are really unique in IWD2, different spell list even, one which offers essentially no direct damage spells, but is excellent for crowd control. Take Greater Spell Focus Enchantment maybe, and either sing or crowd control enemies. Bards aren't powergamey, but they DO contribute.
Beware multi-classing Paladins and Monks, and anyone you intend to cast spells with most of the time. Paladins kinda stink, but Monks are okay, though monks should hesitate before dualing to anything but Cleric or Druid. Druid in particular is pretty cool, as Wisdom is key for both, and Druids remember can Wildshape! So you can let physical score slide a bit if you have 2 levels of Monk planned. Some Monk abilities function while Wild Shaped, which is cool, if not exactly overpowered. Most importantly, your Monk AC bonus works, and you can use expertise or Power Attack in Wild Shape, so this is a solid trick to use for non-HoF. In HoF, you'll get eaten alive if you try this I suspect.
Oh! Last point, If you use characters with light or no armour, one sorcerer can cast Armour many times on each. Monks get a HUGE boost from this, and your other casters probably will like the bonus too. Heck, a Rogue would almost certainly benefit even, so its a great first pick for a Sorcerer in the right party. Stoneskin is a good buff they can cast too, should be your first 4th level pick for a sorcerer, makes your front liners SOOO much tougher.
