I'd recommend several fill/drains with distilled water until you are sure all the green coolant is out. G12 mixed with other coolants is more likely to form a gel/sludge than dissolve any gunk left in the system. And as long as you are going to that much work a CLR treatment would not be a bad idea although once again, I'd be sure to flush it out completely before adding the final G12 mix. Last but not least, you need to follow the 'right' procedure to get all the air out of a ATQ coolant system so be sure to read up on that when you are ready to go.