Drawing conclusions that the Zerg are the Silithid would be like assuming that Kerrigan not the Trade Princes' in Undermine lead the Goblins since her picture is in Goblin Shredders.
The Silithid are native to the world of Azeroth and were part of the Insect Empire that fought the Trolls in the past. The warring Troll factions allied and defeated the Insectoid empire which then was driven into 2 seperate areas of Azeroth. The Quiraj who controlled the Silithid were driven south into the barren land of Silithus where they were trapped and sealed into Ahn'Qiraj by the Trolls. And the rest the Nerubians were ran into the northern lands which would later be Northrend with the other city of Azjol'Nerub.
And the Zergs only insectoid creature were the Drones which were Gashyrr Wasp from the planet of Eldersthine. The rest were actual animals that were assimilated from other planets in the universe that were changed to better suit the needs of the growing Zerg.
Got the info on the Drone(and other assimilated races) from:
http://www.sclegacy.com/encyclopedia/bestiary.php#g2
Then theres the actual Zerg Storyline:
http://www.sclegacy.com/encyclopedia/ra ... ries.php#z
Hows that for someone to argue with, Considering I argue with enough people on the Raid forums when it comes to this games lore lol :p
As for the Silithid Hive in outlands, well I am not sure how to explain that as there are Troll Ruins there also and well the old Troll empire never once entered Outlands as the Dark Portal was opened much later. So I am not sure about that maybe its something Blizzard plans on using for something other then Outlands.