Pablo - All the machiens come from Skynet. Whether they engage in combat is only a matter of who programmed it to do what. Remember, one of the themes of the original Terminator films is that machines are malleable, and neutral until programmed otherwise.
The T-1000 actually can change from human appearance to liquid form on cue. No time at all.
Skule- A hundred years ago the brightest scientists thought travel to the moon was a mere fantasy and impossible by any measure. Part of what makes the Terminator movies so intriguing is that there is a hint of possibility to them. The machines they create are radically advanced but not entirely out of reach; albeit it would be many decades or even centuries before anything of that magnitude could be produced.
The T-1000 is an advanced prototype, a mimetic poly-alloy (a fusion of different metals). Perhaps it is so advanced Skynet had only built one.