I'm in the trailer park level (4 or 5?) and so far all I've seen are the two types of rednecks (why are we trying to kill each other anyway?), and the occasional mosquito, dog, imp.
When do the aliens get here?

I honestly can't remember, but seeing there are aliens already in the second episode, they should appear around the end of the first.

In episode 1.Theres a part where your at a drive in movie and in front of the screen theres an alien.
Though this game has a pitifully small amount of enemies.The other rednecks are alien clones by the way.

THe imps are aliens..

It looks like they use a lot of the 'enemy' sprites for environmental features - pigs, chickens, tornados, mozzies...