So about the Canon.....

The most honest answer is this...

Whatever the studio (20th Century Fox) or the owners of the Alien IP (Intellectual Property, either Fox or Brandywine Productions) say is canon.

The problem here is neither have made any official announcement as to what is and what isn't "canon" to the Alien franchise beyond the core 4 movies and Prometheus, unlike other studios. For example Disney and Lucasfilm famously announced a while back that most of the Star Wars EU we have all read over the past 30 years is no longer canon to the saga, whereas with the Gears of War franchise before selling the rights over to Microsoft Games the original developer Epic Games proclaimed that all GOW material is canon, including the books and the comics.

Canon is:

4 Alien films

Prometheus and it's forthcoming sequel

Fire & Stone comics

Life & Death comics

Defiance comics

Out of the Shadows novel

Sea of Sorrows novel

River of Pain novel

Rage War trilogy novel

Forthcoming Dead Orbit comics

Isolation video game

Weyland Yutani Report

Obviously the films sit on top of the tree and new films may end up contradicting other licensed material.  AvP's canon status is a little murky at present.

So canonically, Predators DO exist in the Alien universe? Do the comics make it so they don't contradict Prometheus?

I dont think AVP is canon anymore because of Prometheus.Here's an example:

According to Paul W.S. Anderson,the director of the first AVP movie, Charles Bishop Weyland was the founder of the Weyland industries.After his death in 2004,Weyland industries was taken over by Yutani,becoming Weyland-Yutani corporation.

However,Prometheus states that Peter Weyland was the founder of the Weyland corporation in 2012 and after his death in 2094,some years later Weyland corporation merged with Yutani,therefore becoming Weyland-Yutani corporation.

Dark Horse comics don't consider AVP movies canonical to their comics.

As for the predators being part of the Alien universe,I would say no.The xeno skull at the end of predator 2 was just a easter egg added by 2 guys who worked on Aliens to see how the fans would react.Since its a joke it shouldn't be considered something as a proof that the predators are in the same universe as the xenomorphs.

And the book weyland-yutani report,which was published by 20th century fox (and they are usually really strict when it comes to publishing books based on movies) doesn't mention any of the AVP movies in the Alien timeline.

So I think that AVP movies/comics are just crossovers.

I agree with Dark Nebula that AVP series  are just crossovers, you shouldn't worry about it.

Alright, thanks guys for your insight!

"So canonically, Predators DO exist in the Alien universe? Do the comics make it so they don't contradict Prometheus?"

Yes.  I can't imagine you'll get a whiff of them in any upcoming films, but yes they exist in the comics that spin off Prometheus and Aliens, as well as the Rage War (which is set in the distant future).

"Dark Horse comics don't consider AVP movies canonical to their comics."

It's not Dark Horse's call.  Fox has a list of what is and isn't canon that is available to licensees.  Of course Dark Horse hasn't produced anything recently that has any need to reference the AvP films, so it's kinda moot.  And given the general antipathy to those films I don't imagine they'll be touching that time period any time soon.

There really is no CANON.....

They are all Canon down to each person and the Studio intends us to maybe see AVP as being Canon in a Alternative universe perspective of it all.

So it all starts with Alien and it is the CANON of all Canons and then movies and spins off branch off from this... some are intended to be more True Canon... but they are all relevant.

You will see maybe what i mean when Alien 5 comes out... if it does... where we get to the End of Aliens... and get a..

What if this (Alien 5) happened instead of that (Alien 3/Alien R)...

So it does not mean Alien 3 and Alien R will then not be canon they would be like alternative....

But if the OT Question was Predators in Alien Universe the answer has to be not as Canon... but as a alternative yes... unless Alien Prequels chooses to throw in clues or links...

As far as Predator... well if we consider Predator 2 as Canon then yes clues are the Xenomorphs are a species the Predators had hunted... but this could had only been intended as a Easter Egg and so again not Canon.

So as far as Alien Canon... then i think SM is maybe the right person to consider as they have worked with those who had discussed what can/can not be Canon in Books etc.

Ultimately its down to choice.... and the Ultimate Canon is maybe how things are shown on screen....

tl;dr: Just check out the second reply, by S.M.

There is really no official canon, but most of the community decides to consider canon the Alien content that follows the story line of Ridley Scott's current trilogy.

AvP films and videogames aren't canon anymore since Ridley Scott obviously chose to ignore them in the new trilogy. Fire and Stone comics and the rage war trilogy (books) though, implement the existence of predators in the series + taking in count the new canon set by the currently ongoing Ridley Scott's trilogy, so my advice is give those a read.

I only consider Prometheus, Alien Covenant, and Alien directors cut canon.

It's such a mess at this point that the canon is more "pick and choose". Yeah, 20th Century Fox and RS dictate what is or isn't canon to a certain extent, but until the sequels begin tying stuff together (and they allegedly are since Scott commissioned a new rule book to be written during the filming for Covenant), we're left grasping at what already exists, and even that can be confusing. A lot of people (myself included) choose to ignore stuff like Colonial Marines, 3, and Resurection. For me, anything having to do with Predators does not coexist with Alien's primary timeline, so there are certain books and comics that I avoid, like the Fire and Stone and Life and Death storylines. 

@tdby Well, it's been revived.