After some thinking, I've decided to go with the custom party after all. I aim at maximum combat and conflict, so it'll be kind of an "evil" play-through, does not really go well with the standard companions.
Here's my approximate plan, still figuring out the details and maybe some flavour for each character.
Rogue protagonist, key roles: DPS on single tough opponents (sneak attacks, death blows, etc.), manoeuvering, disengagement tactics.
Monk: the lashing build from the forums, tank + DPS + scrolls
Barbarian: CC, probably a heavy-armoured dual-wielding tank
Cipher: CC, ranged, perhaps a quick switch build for burst focus generation
Priest: buffs + ranged damage
Wizard: blast build + ranged damage
Barb and Monk will hold and clear out the crowd. Barbarian, Cipher, Priest and Wizard have good synergy at CC. Three dedicated damage dealers that will waste the mobs.
I am specifically trying to avoid "traditional" high deflection tanks such as fighter, chanter or paladin here in order to try and rely on synergy between party members' abilities. Plus, the monk build looks incredibly interesting to try.