The least number of classes that you can have the better. That way you'll all level up faster. IWD is very much different than BG2, as far as I can see, in that Bards are really good with HoW/TotL installed. Also, Druids have the 'Static Charge' spell. It's so annoying that BG2 doesn't!
Paladins are a tricky issue, in that people regard them as having the skills of fighters
and clerics, but not really being one or the other. For a rather good mention of Paladins, look at this
thread, if you haven't already.
If you're going to fiddle about with your party, you should really have a Gnome Cleric/Illusionist. Those guys rock. You'll also want that Thief. Really, as the Vale has some nasty traps, and as Peytin says, it'll only get worse. Run a single class through the game, and when s/he gets some levels under their belt, import them into your existing game. That way, you won't have to start again.