is there a crucial difference between the placement of the c pawn against d6-kingside fianchetto positions (KID, modern, pirc)? the pawn on c4 seems stronger as it pressures d5 even further but Nc3 is also frightening since it prepares e4 faster. 

But I notice that kings indian defense positions almost never have Nc3 before c4, always c4 immediately after Nf6. playing Nf3 first stalls a bit, which is what I prefer. 

One thing just popped into my head now. does a pawn on c4 mean a queenside attack while an early Nc3 means a kingside assault? (like e4-f3-Be3-Qd2-Bh6 idea). c4 opens up even more space on the queenside but rushing to affairs on the kingside might leave it vulnerable to attacks. 

what are your ideas?

The reason you see c4 in the King's Indian is that it really isn't a true KID without it, the typical move order is 2 c4 g6  3 Nc3 Bg7, etc.  c4 is a normal second move in most d4 openings, and is usually objectively best (if only slightly).  If White tries 2 Nc3, Black can always just throw in 2 ...d5 and White won't have much to brag about, it will take a long time to enforce e4.

The Pirc is quite different, since the usual move order of 1 e4 d6  2 d4 Nf6 gives White no time for c4 because e4 is under attack, so 3 Nc3 is the most logical and flexible developing move there - and since White already has e4 in, Black doesn't have easy equality with ...d5 any more.

I like how your thinking about things. 
Think about this:  the opening is about active piece development so you can generate a middlegame plan.

What piece is effected by c4 and how does this effect middlegame plans?

Note too that Nf6 stops e4 which also has an effect on the position and possible plans.

Piece moves are inherently more flexible than pawn moves

The King's indian is a very percise opening idea that puts a stop to kingside action by mechanically blocking the e4-e5 advance with a pawn. The pirc cannt really do this as effectively due to some tactical issues. play around with move orders and you will see why a lot has to do with the access to c4 for the light squared bishop.

study the 'old indian' 1. d4 d6 2 c4 d5 and look at older games and then look why it hasnt been played too much recently. (its fun and educational historically to find the key game where masters went Oh right thats not fun to play.)

d4 sideline players will often use Nc3 to steer the game away from indian defenses. The Pirc is often undesirable for KID players that want an aggressive attack of their own, so the 2...d5 that Estragon mentioned is the typical response and is what white is after.

Take a look at this position and see what the continuations GMs use. and Why? (it has to do with c4) these are the types of things that are very interesting to me. How pieces belong on squares not pawns. Its like sharing a toy with a baby brother you have to do it but you should be really careful about which toys you share unless you want them broken or not to see them again. you have to share squares with pawns but care should be taken

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