You can basically run any processor into a PA. The sound may not be optimal as a guitar tone, unless your processor has amp-model/cab-sim features or just an equalizer tweaked the right way.
Yeah. Usually I don't use anything but Line Out for that. Tried it with amp out to a keyboard amp and/or the direct box to the sound mixer, and it sounds worse that way in my opinion.
I've tried my GT-3 to some P.A. every now and then but I haven't really ever been impressed by the results. My PodXT has definitely worked better in that sense.