First of all, thank to all of you for the efforts you made for you detailed replies.
I read it in a begginer guide from the site gamepressure. I quote you the part in case you are interested: "More isn't always better. You don't need to expand your party to six members. If you think that you have created a perfect 4-person team, then don't change it. Gained experience is split between all party members. The less members, the quicker they will level up."
As you explained earlier, I share this mania of doing every single quests and kill every single mobs too. So I'm more inclined to believe you when you say the splited experience won't be an issue for a long run. However that makes me wonder of something. I have set the difficulty to normal for my first game and you seem to believe that even at the maximum difficulty you can clean the game with a four men party. Thus that makes me think a six men party in normal mode could reduce drasticaly the difficulty of the game.
So I think I will stick with the four men party, I always liked the D&D or the "D&D like" games for their difficulty and I don't want this game to be too easy. (But I think I must clean the normal mode a first time before trying the hard mode, this type of game need us to have a good knowledge about the game design and the mechanics before trying harder difficulties.)
Plus if I get bored because I reached the level cap too quickly, I can always create a new level one rookie at the late stage of the game. (That could be fun to struggle trying to make survive the poor rookie during a fight where the full level max geared/pumped-up party would crush some demons/elementals/beholders ect..)
My mage is already specialized in the Lore. So for the mechanics I think I'll choose a dexterity based character for roleplaying purpose.
As agreed earlier by Theurgist, I will use a Paladin for the tanking issue, the Mage for CCs and the the Druid for killing everyone who had the misfortune to stay close to eachother.
So that gives me an open space for either a Thief, a Cipher, a Ranger or a Monk. I'd like to use my last character as an off-tank, more to avoid my casters to be surronded than to really be able to tank everything. I've heard the Monk could be a good off-tank but I've also heard the Ranger have his famous animal companion wich could fulfill this role at the early stage of the game. The cipher don't appeal me that much, a fighter illusionist thing seems a bit overpowered based on your description. I'll make further more research about these two classes and I think I'll end up with a well balaned team. Anyway, I want to thank you again for your advice and if your want to add anything else, don't hesitate. (; Frouwenlop