There's one gun that the Combine use that are from their planet, the AR2. The reason their vehicles are vulnerable to earth ROCKETS is because if you were to shoot a loving tank, it would probably blow up immediately, thus meaning anything can blow. (dummy)
Now the reason they use the MP7 and the Glock thingy, is because they're using our technology to see how we use our weapons on enemies, and probably steal some ideas. But they're also cut off from their world, they use a dimensional sling shot to get to planets, they have a pretty good probability of getting there, but they could always get forgeted and sent straight to mars rather than earth.
The reason it was a seven hour war was because of the majority of troopers they have sent into earth. Earth's military forces were already dealing with the Xen creatures and now had to fight the combine as well, but the combine are more advanced, more intelligent, and have a greater military power. Earth's forces could easily deal with the oncoming Xen creatures, but with the Combine closing in as well, it was just impossible.
While on Earth the combine simply took a lot of military personnel and turned them into Civil Protection. As they did this with all surviving Earth militia. They even did this with simple civilians but not many.
