[@StoneWolf] Maybe five would be a good, solid foundation. Or maybe I love prime numbers and am biased.

If so, I'd love to go for comms, someone who is actually interested in learning the aliens' language despite the fact they're trying to blow our faces off.

Plotwise, maybe initially there was a tentative peace. Perhaps they are a nomadic race that f*cked up their own planet and are now traveling from star system to star system (sound familiar?). 

They discover Earth, with its "world peace" and "let's be friends" but after some racial tension and a baaaaaad incident, our visitors decided to be permanent residents. The ONLY permanent residents.