[Radek]

1. First, your new party should not be a party of six. Otherwise, you'll be unable to accept Imoen into your party in the Irenicus' Dungeon. Note that you cannot kick out any of your custom characters - sending a custom character away equals losing the character FOREVER. The game cannot re-hire a custom character!
Next, the custom party is a party which fits your play style optimally. Your characters will be much stronger because they will have much better stats and they will cooperate much better than the game characters. A party of six custom characters will turn the game into a massacre.
Therefore, I suggest a party of 4 or 3 custom characters.

2. Consider what you will need in the game:
a) A thief, which will detect and remove traps. It need not be a pure thief. It can be a dual class (the thief part is here only to remove and detect traps) or a multiclass. It depends on your play style. If you like backstabbing, then the assassin can be a trap detector/remover simultaneously.
b) A mage. There is a lot of situations when you will appreciate a mage in your party. A mage can cast Mordekainen Sword, Invisibility, Maze, or protection removing spells, for example. A sorceror or a dual class something/MAGE is recommended. I consider the best choice a kensai/MAGE.
c) A priest, which will heal and cure the party and cast defensive spells. I recommend a dual class something/CLERIC. I consider the best choice a ranger/CLERIC.
d) A warrior. This guy is here to make use of good weapons and armor in the game and show your enemies that a good sword is something that has to be respected.
How will these professions be implemented in your party depends on your play style and on your preferences. For example, an assassin, a kensai/MAGE, a ranger/CLERIC, and and some kind of a paladin. Such party is able of all needed actions and makes use of Carsomyr. It contains four warriors, one mage, one and half priest and one thief. Or you can replace the paladin with another assassin, if you really like backstabbing. In the example above, you need to create an assassin (or two assassins), an intelligent kensai, a wise ranger and a paladin. Naturally, there are many other combinations that will do equally well.