piranha1: Is rogue/wizard a viable combo? To be more clear, I'm thinking to pick rogue for skills, both "thievish" (locks, traps, etc.) and "social" (diplomacy, bluff, intimidate) and wizard for the combat part (with Intelligence being a good start for wizards, that should give him plenty of skill points).
You won't be able to get all the social skills without hobbling yourself. Rogue/Wizard has no synergy beyond skills, so you're hobbling yourself in every other way. Rogue is already the weakest non-spellcaster in combat so taking Wizard levels exacerbates his greatest downside, and Wizard spellcasting falls behind incredibly quickly if you multiclass so taking levels of anything else hurts. As kmonster mentions 1 level of Rogue is doable; don't take more than that.
General rule of thumb for Wizard or Sorcerer multiclassing: 1 level in another class is a sacrifice, 2 levels in another class is a steep sacrifice, 3 levels in another class makes you borderline incompetent at wizardry/sorcerer, and 4 levels shouldn't even be considered.
And how important would Charisma stat be for him, would it be okay to leave it at 10 (so no penalties) and get all the social skills from well, skills, and not stat modifiers? (I was thinking of starting stats along the line of Int 18, Dex 14-16*, Con 12-14, and leaving the rest at 10, that should be doable with the starting stat budget, if I recall)
10 charisma should be fine.
Also for another character, I've read in some earlier post, that a paladin is good for saving throws if you have good charisma, so would level 1 Paladin/rest Sorcerer be a good idea? (I guess crossing them with a bard would be difficult if not impossible, due to alignment conflict - can alignment in IWD2 change, the way it's possible in NWN, and if so, would the character lose his paladin abilities, becoming a Fallen one?)
I ran a Paladin/Sorcerer in my last IWD2 run. It works, but it's kinda underwhelming. At low levels I got some utility out of being able to wade into combat with protective spells, but the party barbarian was cleaning house without the paladin/sorcerer getting a chance to do much of anything. At higher levels it was just a Sorcerer with slightly reduced spellcasting and massive saving throws and I had little reason to ever bother with melee.
*So he'd be at least decent with a bow or so, when not casting.
Just be an elf; they get proficiency with longbows for free, no multiclassing required.
(I thought of just picking a bard for social things, but Intimidate isn't a class skill for them I think - unless it's possible to use skills of different characters in the same conversation? If so, I could take Intimidation on a barbarian or so, if it's a class skill for them like in NWN, haven't checked yet)
I don't recall ever having much use for intimidate. Conversation skills aren't used too heavily in IWD2. Nice to have, but I got by just with diplomacy.