The 650's don't link up at all with the G600. Am I right in saying the GTN750 is fully compatible with the G600?

Not sure what you mean by 650's..  The GTN 750 operates quite nicely in this airplane.

 

Best done with the steam gauges though, since the G600 does not pick up the GTN flight plan.

It was poorly phrased 2 questions in one. I'm now aware the GTN650's don't work at all in the panel besides being 2d popups and both the GTN650 and 750 don't link with the G600.