Why was Gandalf so sure that Sauron & Smaug would help one another, given the chance?

Did you ever find anything definitive on that? I don’t recall seeing such, I just got the strong impression that he was leaning that way from the too many books I have partially stored away in my brain.

You’ve come to the right place, then! Qadgop, What Exit? and others here can teach you all you’ll ever need to know.