I have descent and gears of war and...especially with gears of war i thought...what if?!

What if the side who wins the Imperial Assault campaign lifes for a few years of "peace" untill a new alien races emerges...the Locusts!

The Decsent monster could be an addition only if they don't carry any sort of weapon(s). Those weapons would surely be predated to the Star Wars era but I don't see a problem facing a Shadow Dragon guarding a certain artefact or Wraiths haunting the Tomb of a Dark Lord,...

I think they would be a good fantasy addition to it. Not really sure anymore if the sizes of the miniatures are the same.

I was thinking of using some of the Star Wars miniatures from Wizards of the Coast for the purpose of identifying the elites and such.

Yes, made some Gears of War custom cards for the miniatures and everything.

WOTC pieces work really well. Everything is basically the same scale. ATST are the exceptions. WOTC is 3x3 FFGv3x2

The WotC Probe Droids and E-Webs also have different bases than the IA ones. But other than that (and the ATST, mentioned above), I'm finding that the WotC minis are a perfect fit.