...anyone seen Dreamcatcher?  LOL!  Well anyway its sort of along those lines.  A few humans working with "Duddits" to destroy the evil Mr. Gray...

Yes, I saw Dreamcatcher.  That's right, I think that at the end there was a "positive" focus alien (which was disguised as something else) that neutralized a "negative" focus alien.

Plus telepathy, and entering people's minds, and Morgan Freeman saying something like: "You hear those voices in your head -  this is not a typical adversary."

hehe, forgive me, it's early.. well, it *was* somewhat fun ;-)

"But the one scenario that as far as I’m aware has never been covered is the concept of mankind joining forces with one alien race in order to combat another."

There are so many story lines, that it seems to me that this one has most likely been covered in literature/media.

Any suggestions, anyone?

Even so, one "archaic" possible example that I can think of is "Lord of the Rings" where the elves and dwarves and positive forces of nature join with the humans in order to deal with what was spreading from Mordor.