These are my own headcanons for Mixels:
The Spikels are actually the first mixels to live outside the water and also lived on land too.
The Flexers once lived underwater but throughout time, most of them now started to lived on land or outside of water.
Mixel Land has now been shattered, destroyed, and taken over thanks to King Nixel, Major Nixel and the other nixels during the events of the Quest Of The Mixamajig but someday, the Mixels might as well put Mixel Land back together once again.
The Lixers are actually living fossils and the others once thought that they were prehistoric relics of the past but the Lixers still understand the advancements of Planet Mixel just like the other Mixels.
The Orbitons and Glowkies were once competitors back then before they moved to the Mixel Moon.
The Fang Gang Mixels are mostly primitive while some others are advanced.
The Nixels want to track down everything the Mixels do.
Nixel Land has lots of laboratories, factories, underground railways, and power plants which are found in some areas and regions of it.
These are all of my Headcanons for the Mixels now but later or sooner, i might need to think of some more headcanons for now!