Starting a new game, have played as 6 man party, 4 person party and solo. Looking to start with one char then add 3 others along the way to keep average levels down. Probably going with basis (some mix ins possible) fighter, cleric,rogue, socerer, druid. But who starts? I think cleric, can go melee early, small self healing, then get levels up early to get animate dead, etc. Should soc go next for good spells earlier? When do you think you need some beef up front and when do you really need rogue for traps, chest, etc?
At what points should you add, at each chapter or earlier?

I usually go with Cleric or Paladin first when I playing using this tactic. I usually base it on who can survive the goblin tunnel best (always tough for a weak level 1 soloer) and who I need to have levelled up ready for the Palisade battle (one of the toughest encounters in the game, in my opinion).

I too go for either Cleric or Paladin for no other reason than the 300XP you get for healing that glass-arm soldier :D At any rate a Human can easily get to L3 in the docks itself.

I generally add the DG Rouge somewhere in Targos since it leads to added XP, items and the high AC sheild is always useful. These two will clear out the bridge and the fort too.

An arcane spellcaster (most often a Sorcereror a Bard) gets added after the airship crashes. The Icewall and Ice Temple are great XP boosting areas and you can always find ways to keep this new charecter out of harms way.

The fourth charecter is added whenever I feel that I am not getting as much XP as I should.