The only thing I'd modify is that there's actually 3 groups of aliens.
The first group(Skitters) insectoid/biomechanical and are the main aliens we see each episode.
The second group(Overlords) the very tall (like 14 feet tall) bipedal creatures who control the Skitters. They don't appear until the season 1 finale and then have some appearances in S2.
In S2 we find out there's a Skitter rebellion and that they were enslaved by the Overlords when they came to their planet. Some Skitters developed the ability to resist the control of the harness and then began the rebellion.
In the S3 finale we have the third alien race (guy in powersuit and space pod) land.