The old EU left hints here and there about other life forms, but they were mainly one-time mentions, a way of saying "they exist". A few were lucky to have a comic or a short story based on them such as "Aliens: Reapers" and "Aliens: Alien".
The recent Fire and Stone comics and the Titan Books trilogy explored a bit more on other life forms. The book "River of Pain" mentions there are benign intelligent races, most of human encounters with them weren't all that disastrous. And yet, there are hostile ones and Colonial Marines have some horror stories to tell of friends that they lost during such encounters. It also mentions that the Arcturians are trading partners with humanity.
And then "Sea of Sorrows" mentions that the Arcturians were the first alien race encountered by Mankind and thus answering years worth of debating on what exactly "Arcturian Poontang" really is.
Fire and Stone also shows a Predator go on a hunting trip across the galaxy and highlights his key encounters with hints of the Engineer's existence.
I remember people being concerned that introducing too many aliens would make it very similar to Star Trek/Wars or Mass Effect. Some fans like the universe in ALIEN to be dark and gritty.
I believe it can still maintain that isolation. Despite all of these encounters, humanity is still very much alone. I don't see any "United Federation of Planets" or "Empire" or "The Council" in this universe. It leaves a hint that every race is pretty much on its own for the most part with some interactions here and there.
The old EU may raise some eyebrows onto why most of the races are humanoid but ever since the Engineers were introduced, humans certainly weren't their only creations. That leaves room for the other aliens to have similarities and yet be different. Like the four armed red skinned primitive aliens that the Predator hunted in the first panel of that image, then the bug like things (heh, "Bug Hunt") in the second panel and I have no idea what that armoured thing is in the third.
Apparently Predator South China Sea mentions an alien life form whom the Predator jacked its hover board from while hunting on its world. But I haven't read that novel.
What do you all think of this addition of more races? How does it shape the Alien and Predator universe for you?
I think intelligent life would be pretty rare, especially space faring life. Even if there are hundreds of races, that's still a tiny number compared to the billions of star systems in the Milky Way alone.