Is this to imply that we may be related to the Xel'Naga in some way? Doubtful. The 'Naga originate from a galaxy other than the Milky Way. And too, consider this: The Overmind assimilated and destroyed a fair number of the Ancient Wanderers. Would it not know the Xel'Naga when it met them? In the manual, it says that several Zerg probes discovered Humanity at the end of the galaxy, not Xel'Naga. And too, there are the Protoss. Would they have not been able to identify the sires of their race?
They would gladly nuke the surface of a planet of buglike creatures that went berserk
The 'Naga never touched the world of Zerus. When The Overmind discovere the Xel'Naga fleet, it severed its connection to these Wanderers, and made to attack the armada. As far as we know, there were no weapons fired by the 'Naga; they fell quickly and easily to the Swarms.
What would make the zerg go crazy and attack the xel naga?
A number of things. As you suggested, the Xel'Naga may have possessed some information that could easily counter the Swarms; thus the Overmind saw fit to destroy them before they could mount any kind of counter attack. Or perhaps the Overmind was merely following its instincts and wished to assimilate the knowledge of the Xel'Naga into itself. It got a lot from this infestation, as it was able to process the information of the Xel'Naga, as well as incorporate the Khaydarin energies into itself.
(However, it is evident that the Overmind was not able to assimilate
all of the Xel'Naga knowledge, as it could not find the Protoss homeworld until it tapped into Zeratul's mind decades later.)
but i think the terran play a bigger part in the whole entire sc universe than can be known. Humans have something the other 2 races DONT have. Freedom. The zerg must obey the overmind. The protoss must obey the abdicators(right?)
Well, yes and no. Though we possess "freedom", it doesn't earn us much, as we continually fight amongst ourselves. Even with the advent of the Dominion, much time and resources have been wasted. We don't have the abilities necessary as of yet to gain a real foothold in the SC Universe. When you look at the Great War for Domination (the UED's term for the war chronicled in StarCraft) the fight is really between the Zerg and the Protoss, while the Terrans are stuck in the middle, merely trying to survive. (Of course, with all armies in the Koprulu Sector spent, the UED had a fairly easy time coming in and whooping as with their fresh forces. However, since they hadn't before experienced war on this scale, they fell relatively quickly, for they did not possess the experience or dynamicism of the Dominion warriors.)
But back to the topic at hand- freedom. The Protoss were indeed restrained by the Khala, as it was only a superficial band-aid for the scars and follies of the Protoss. It may be that the Communal Link has lost its place in the Protoss mind of priorities, and the Khala sort of forces it into the forefront of one's mind. Nevertheless, with the various tribes reunifying, this may no longer be an issue. (The Khala also set up a definced discipline of Psionic progression, so it wasn't all bad.)
Finally, the Zerg. The Overmind was the incarnation, the very manifestation, of all Zerg thought and consciousness. So in a sense, the Zerg still possess freedom, merely "centralized". However, with Kerrigan having taken power, the Zerg link has been weakned. Do you not find it odd that Kerrigan was not particularly nervous when Duran went missing? He was part of the Zerg consciousness; that just can't happen unless something big is going on...
The Xel'Nagians (Or whatever they are), didn't do jack, as far as i know.
They gave rise to the Zerg swarms and mighty Protoss Empire. What else do they need to do? Create weapons of mass destruction?...
*looks at the temple of shakuras*
Check and mate.