no. they go to war.Doubt it. They are extremely xenophobic and are single minded about destroying humanoid life in the Milky Way Galaxy. In Star Trek Online they were manipulated by the Iconians by them sending pieces of gateway technology into fluidic space. They may have stopped attacking when they discovered that the Iconians were manipulating them, but they would likely return to fluidic space and only come out if threatened. If no one travels into fluidic space, they could stay for centuries or even forever.
Species 8472 isn't even from our dimension, so no.
There is no canon source that discusses events after Voyager. However, VOY:In The Flesh leaves relations between humans and 8472 on a positive note.