If Leah and Rosalie met or talked for the first time (not sure if they actually met each other or not), what do you think may happen?

There are some possibilities one of them, and in my opinion the more creative, they would start to say that bella is an outsider in the world of the supernatural, them they will see how alike they're and they will be friends.
They are more alike than they seem, so they can be friends!
Leah hates being a werewolve and Rose hates being a vampire!
Rose seems evil, but she tells her story and help Bella, Leah seems evil and she tells her story!
The both story end badly!
Rosalie's boyfriend (royce) raped her and Leah lost her father!
Then Rose turned into a vampire and Leah into a werewolve!
Rosalie helps Bella with Nessie! And leah helps in the volturis fight!

I'm 100 percent sure they would attack eachother knowing them both they would kill eachother

there would be a bitch off!

well...most of the people find leah and rosalie the most likely ignored so maybe they'd be friends?

after one minute Rosalie would pull Leah's hair out and say, "girl, you KNOW i'm prettier than you!!" lol.

I think that there would be an arguement, ofcourse!
I personally think that Rosalie would call her a "dog","mutt"etc. Sorry if you dont like it but thats the character Rosalie is :) aha.

I guess that is hard to say. I guess if they actually sat down and listened to each other they would have a lot in common and be able to relate to each other. That's not really their personality though, so it's hard to say.