You should really watch Aliens again, your memories seem to be confused a bit. Or maybe you just didn't look into the movies that deeply; I had to, back when were making a trailer for Colonial Marines (but it's never been completed or released). In fact you should re-watch all the movies, the first two are still quite good and the third isn't so bad either
So, the first alien was designed to look as much different from a "man in suit" as possible. The guy playing the creature was 7 feet tall and extremely thin so he could usually look strange enough on his own. Sometimes they also had him squat and crouch, but the most important trick was to hide most of the body in shadow or outside the camera's view.
The "Aliens" creatures are all just humans in very primitive suits, they were dancers and acrobats. But there are many clever tricks to sell the illusion that they're inhuman - hanging from wires, flipping the camera upside down and so on. Also, the tubes on the back of the creature and the tail itself really help to confuse the silhouette, especially with the quick editing, fast movement, and clever lighting. They're also flopping around a lot because of the costume's simplicity And of course Cameron was clever enough to keep hiding most of the creature in shadow and smoke the same way Scott did.
The suits were this simple:
By the way, the dome was ridged because all the action scenes would have broken a smooth dome in minutes. The ridges reinforced the costume enough to withstand a little beating. Some of the surface or skin details were also a bit more organic compared to Giger's "biomech" design.
The Alien 3 "runner" or "dog alien" is inconsistent in the movie - some shots are played by a man in suit and there it's constrained to human anatomy. They try to hide it as much as possible, but the limbs are shorter and the entire creature is much thicker in these shots.
Full body shots are created with a stop motion puppet which runs on all fours, has digitrade back legs, and the four tubes are gone from the back. It is actually very different from the creatures in any of the other movies.