A Station Eight Fan Web Site
Three races are involved. The first is Nokkar's race, the N'Kai. The second is the Space-Spawn. The third has not yet been revealed.
Nokkar's race is older.
No. Nokkar simply assumed that, since they weren't human, they must have been extraterrestrial and hostile. (Rather similar to the immediate human assumptions about gargoyles, isn't it?)
GargWiki.net has answers for all your Gargoyles questions.
Includes episode commentaries by co-creator Greg Weisman, interviews with the cast, and a documentary on the fan convention.