First of all, 3rd edition is a little bit stranger to me, I'm a 2nd edition man!

I'm confused about character generation and feats. There's cool feats for mages like spell penetration, maximize spell but they cost 1 slot more! For ex, I can cast 4 fireball but if I maximize them it's only 2. isnt it a bad thing? I'm thinking about 1 paladin (as leader, so I'll max his INT WIS CHA) 2 tank 1 dwarf fighter 1 human fighter. 1 cleric 1 mage and 1 thief.

should I multiclass my fighters with few rogue levels? can you give me some advices? thanks

Do NOT, I repeat DO NOT use a paladin or monk as party leader if you ever want to get paid for anything.

As for the maximized spells I'm not sure what your talking about as their is a feat in 3rd edition that I don't believe is in IWD II where you can memorize a spell as a higher level and then gain more damage for it. However, you do have spell focus which allows you to give ppl minuses to resisting your spell and their are the feats like Aegis of Rime, which allows you to due extra damage with an elemental spell and gain resistance against that spell. Read the text closely on the feats because you'll notice if you can take it more than once it will do roughly the same thing again, but just give you that much more of an edge.

I was also new to 3rd edition and had to read the IWD2 manual pretty thoroughly to acquaint myself with the ins and outs.

First of all there is no big deal in having the Paladin as a leader (main talker) as long as you use another guy (the cleric ?) when you claim the reward*

Dwarf fighter is ok. I would take a fighter4/BarbarianX instead, since you already have a fighter.

Human fighter is ok, you can take a single level of rogue if you wish, then you will not need a pure rogue and can take a druid instead of him.

I got a problem. If anybody can help me about it, I'll be glad. I noticed that the social abilities of the leader is important. I mean CHA,WIS,INT. And the leader should be a strong melee character because he's on the front! But if I raise these stats (CHA INT WIS) he's lack of STR CON DEX. any idea about it?

My Leader is an Asimar Paladin with the following starting stats:

str:18
dex:10
con:10
int:10
wis:12
cha:20

Charima is important for paladin's skill so they make the best character to use when talking to people.

But you don't really need high charisma. Just spend your points into Intimidate, diplomacy and bluff.