I don't think any of what you said excuses them being a mindless entity.  Their actions if interesting imply some thought process.  If they carry out ritual or such and behave in unique ways, go for it by all means.  Physical expressions and behaviors can be very interesting.  But so far it doesn't sound like they're going to do much other than be basically a weapon for another civ.

If their sole purpose is to be invaders, why does it have to be a NPC?  Surely someone here has a alien species that would be more than happy to invade you.

But really my concerns is that their status may be elevated and that they'll not be the regional issue you envisioned them as.  Since if that's the case than they need to be more interesting than what you intend them to be.  If not, than I apologize for all this scrutiny.  I just don't want this to be a shooting a RP where all everyone can do without dying is shooting bugs in space.