Wow! Lots of replies, and interesting to hear what you guys think.
My own thoughts are that the 3rd space aliens wouldnt necessarily be too powerful for the younger races, as was seen in 3rdspace the film. The whitestars (and to an extent, starfuries) were a match for their raider sized craft, and besides, if the ancients could feature in battles against the younger races, why not these gribblies?
I've no idea how hard their ships would be to model, but I trust what people say when they reckon it'd be tricky. I'm sure mongoose will think of a way though, some of the more exotic alien ships must have been hard too.
With regards to background, I think it would be pretty easy to come up with a plot device to bring them into ACTA. Sure, the vorlon's portal was destroyed, but if these beings are so advanced and can give the vorlons the heebie-jeebies, then why cant they build their own portal so they can come get us? Certainly, the fact that our galaxy is still populated suggests there may be technical reasons why the 3rdspace aliens havent done this already, but perhaps they can manage a wee visit now and then. If all those dead worlds encountered by the Excalibur are anything to go by, I reckon there's been more than one genocidal race in the galaxy besides the Shadows (or the Vorlons).
Anyway, I've had my rant :wink: