Curt Connors and Flint Marko were taken over by symbiotes almost immediately. Both were successfully parasitized and transformed into humanoid symbiotes.
This was, of course, due to Riot's misjudgment. He had assumed the two still possessed the abilities of The Lizard and Sandman, so he had placed special importance on them. But Riot had been one step too late. They had already visited the Three-Spider Universe, and when they returned, they possessed only the strength of ordinary people.
The old professor became a burly white humanoid symbiote. He was somewhat leaner than Riot, but most importantly, both his hands were whole again.
Seeing his restored right hand, Curt Connors did not even consider resisting. He only wanted to keep that hand forever. Thus, the symbiote encountered no obstruction whatsoever in controlling him.
Flint Marko was rather unusual. Even Riot did not fully understand the principles behind his Sandman form, so he naturally could not guarantee that a symbiote could parasitize him successfully.
To improve the odds, Riot had two symbiotes parasitize one human at the same time. He had figured that even if one was rejected, the other should succeed. Yet both symbiotes ended up merging into Flint Marko's body.
One was earthy yellow, the other muddy brown. The two similarly colored symbiotes formed a towering, imposing humanoid symbiote.
He was even much taller than Riot, yet he still crawled before the symbiote leader.
However, the symbiote attempting to parasitize Harry Osborn was not nearly as successful as the others. The green slime-like symbiote tried to slip through the gaps in the Green Goblin Armor. It also spread itself wide, trying to swallow Harry Osborn in a single bite.
But whether a symbiote could control its host depended on the host's mental strength.
Take Venom, the weakest of the symbiotes