Dude, Marco is not some random dude.
He was the commander of the WB's 1st division. He ability was already somehow known among the marines. People know about him.
Knowing about your enemies abilities and what they can do is military advantageous.
Don't try to make it sound "oh so unrealistic". It's just basic stuff. One of their main enemy forces are going all out in a war. If you don't have information about them, watching the broadcast is a GREAT way to learning about them.
So YES, they should be "looking at the dude in the bottom left".
They don't need to remember every single fodder they fight, but, again, it's Marco we talking about.
And about the rubber, that's how rubber works in One Piece.
Nami realized Enel attacks didnt work on Luffy, because....rubber. It's not some secret tech or whatever. It's just common sense for people in this world.