I have several different questions

Good questions... See you at the theaters to find out in two years.

A better question: How dangerous will Mothra be to Rodan? 

Her parents work for Monarch, run wild from there 

The girl!

So will Godzilla 2 mostly take place in one place like in Kong Skull Island, or would it be more of a traveling plot like the 1st Godzilla movie?

^

takes place in the modern day.

I meant location...as in what location will it take place.

Given how the timeline has showcased a multitude of different locations and accompanying dates, I'd say that the sequel will likely visit a few different global locations.

As for the OP's questions, I'd say that Mothra and Rodan will be dangerous to Humans much like how Godzilla was: unintentionally causing destruction in the pursuit of some other goal, without any malicious intent.

As for Millie Brown's character, it's hard to say for certain. Given she'll likely be one of the leads, and given Mothra's general character over the years, it's not out of the realm of possibility that she'd communicate with the giant moth in some capacity, somewhat like Kong's relationship with the Humans he meets.

As for King Ghidorah, I'd say that his beef would most likely be with Godzilla for the most part, considering the two have tussled in the past. The two would be focusing on each other for the most part, with King Ghidorah likely being the more chaotic and destructive of the two rivals. I'd say that he probably wouldn't form any real interactions with specific Humans, probably just going on a killing spree of them in general.

What Gman said about Ghidorah