For the mech battle scene, remember they manned the barricades at the school as the first mech came down the street and every one opened fire. There was even a moment in the action where the video showed from behind the mech and you could see some of the shots going clear through but the mech was still trying to advance, but getting slower as it took fire. Then they all cheer, but hear more mech steps and do an ammo count. You hear them go down the line with several stating a couple left, some out etc... Then as several mechs come into view ben let's out the radio burst and the mechs then start wandering away.
As for the new aliens, no doubt they will be friendly, at least in the manner that they are against the current aliens. To stretch the plot of the new aliens landing throughout the next season there will probably be some sort of conflict, whether ego or control based, between the two factions.
Thee real interesting tidbit is that there could be as little as 12 of the true evil aliens on the planet; but with one down you can imagine the rest will be real hard to get to.
The real plot element left out there is why did blondie slip her boyfriend the bug - particularly if all the humans were about to become dead.
