I think the "can't shoot" thing was because they were in a space station (as opposed to a colony, like LV-426) and there were several of them. Shooting one ain't gonna eat through the entire space station. A dozen or two? Absolutely.

Yes. In the 1979 alien when they are trying to get the facehugger of it's face they cut it releasing the acid blood that melts small holes through several floors. They figured out quickly that there was no way they could remove it and had to wait.