Cho'Gath was summoned by summoners in error, something from Icathia stirred causing the summoning to be perverted (they didn't want to summoner Cho'Gath). There have only been two individuals to stir Icathia, Kassadin and Malzahar, my money is it was Malzahar, at the time of his rebirth.
The creatures of the void can enter to Runeterra without help of him
Kog'Maw's lore disagrees with you
When the prophet Malzahar was reborn in Icathia, he was led there by an ominous voice which thereafter anchored itself to his psyche. From within, this voice bestowed upon him terrible purpose, and though Malzahar was no longer tormented by its call, the voice did not cease its unrelenting summons. This baleful beacon's gentle flicker - now fastened to Runeterra - drew forth a putrid beast that ambled across a threshold it did not understand, widening a fissure between spaces which were never meant to meet.
When Malzahar was touched and reborn, he became the beacon and focus of all Voidborn entering into Valoran. This is why Kassadin has been trying to kill him, and why Malzahar wants to recruit him. If Kassadin can break the beacon, the Voidborn need a new anchor, while if Malzahar can have Kassadin join him, their beacon would be that much stronger.
Other aggressive creatures of the Void, like Velkoz or Reksai, would kill and eat a random lesser creature that opened a "door" for them.
The fact is, that he cant expect that all the void creatures will be friendly with him because he is "allied" of them... As someone said "That's like expecting the lion not to eat you because you didn't eat him"
It less, not expecting a lion to eat you, instead, it would be like expecting the Spawn of Cthulhu not to attack you because Cthulhu himself has chosen you to do his will.
These aren't some animals (mostly) from another planet, these are eldritch abominations, and their god has a plan involving Malzahar, and you can bet your right hand that they do not want to piss it off and attack Malzahar. Would you want to knowingly bite the hand of the god that opened the door for you through a prophet?
I would argue that even the more animalistic Voidborn would not attack Malzahar to avoid the wrath of their god, even in another plane, or they can feel it through Malzahar.