I Am Number Four/WMG

The Mogadorians and the Loriens are from the same planet.
Both are very human (though, being villains, the Mogs are less so). Both dissolve into dust upon death. Both come in very small groups. The Mogadorians never invaded Lorien: that's just a cover story. In truth, there was a civil war between 2 factions of their shared race, and that war resulted in their planet's destruction.

All of the aliens are Chimeras.
It explains why they look so similar to humans -- they just shape-shifted into looking like that. They obviously vary in how well and how fast they can shape-shift.

I Am Number Four takes place in the distant future.
The Loriens aren't alien creatures; they're cybernetically-enhanced humans, hailing from the off-planet colony of Lorien. Earth simply degraded into a seemingly modern state in that time. Speaking of which...

The Loriens and the Mogodorians are actually Orphenochs.
C'mon, someone had to say it. Much like the Kaijin from Faiz, many of the aliens  can change into grotesque forms and boast superior strength and capabilities to humans. Of course, the most obvious reason for this is because the aliens disintegrate into dust when they die, much like Orphenochs.
 * Except in Faiz, they were more akin to Zombies, rather than Aliens.

Number 5 is Sam.
His "father"'s abduction is more connected to this struggle than Sam thinks. Years of living with his no-nonsense stepfather have caused Sam to forget his alien origins.

Number 5 is Bernie Kosar/ .
They never said the other 8 Loriens all had to be humanoid.
 * Or he is humanoid in his natural form but his special power is shapeshifting. Six just assumed when he shifted into  that he was one.

The Loriens are Time Lords
They are somehow different from the other Time Lords, so they were saved when Galafray was destroyed. They are stranded on earth because they are missing a TARDIS. The Doctor may or may not be one of the numbers or their guardians.

I Am Number Four... but so are they.
The Mogadorians we saw weren't soldiers or anything: they were the less benevolent counterparts of the 9 Chosen Ones (or what was left of them, that is). The lead Mog could either have been their Number Four, for his being the main antagonist, or their Number Six, for his being the strongest. In addition, each Mog possibly had a unique weapon to parallel and contrast their formers' super-powers.

The Loriens are Exalted.
They have anima banners, mote-sharing charms, and even the equivalent of a Solar-Lunar bond. What more do you want? Of course it's not obvious what kind of Exalted they are.

The Mogs are allergic to something on Earth.
We always see them take what seems like a somewhat struggled inhale through their nostrils. It gives off the impression that they're trying to clear their nasal passages. Or at least whatever passes for them on a Mog. Also consider the way the head Mog talks. It sounds like his nose is stopped up.

'Power of Six' refers not to the power of the Six, but Number Six.
Self-explanatory, really. She's going to be pivotal in a large part of the plot, and she'll develop a bunch more powers while she's at it.

The dusting effect is a result of the disguise technology used by the Loriens, and possibly the Mogadorians.
The technology runs on the users' biopower, and holds them in a configuration similar to the targets'. If the Mogadorians are using this technology as well, they either don't have as much energy as Number Four, the other Chosen Ones, and their guardians, and thus can't change their fundamental biology like their air-gills, or they are more different and merely can't change as much in comparison. The flying squirrel-ogres could be explained as being a transplant from another planet with a similar technology used as life support instead of disguise.

If the Mogadoriens aren't using the technology, it's possible that the flying squirrel-ogres are from their planet, where everything does the dusting thing, and it's either a coincidence that the diguise technology has a similar effect or the chimeras are from there, too, and the disguises are based on chimeran biology.

More simpler the 'dusting' is the result of a self destruct device.
Both sides are at war and know they need to keep a low profile to survive on earth. If they are killed in action there's no guarantee that their allies will be able to retrieve their remains before the authorities get to them. So it makes sense they would have some sort of disintegration implant set to trigger upon death.