i was wondering if it would be possible to harmonize pinch harmonics say on the 3rd fret A would it just be like harmonizing in thrids?
Yep you could harmonize pinch harmonics - they're just notes.
Harmonics are notes just like everything else, they're just a couple of octaves higher. So you can use them in every way you would withe regular picked notes.
Yeah. Sounds too dissonant with vibrato, though.