Oh yes wrt to G13 is okay to mix with G12 also with G12+ and I'd reckon G12++ - I'd doubt if you can buy any G12, G12+ or G12++ now.
Makes me a bit worried to read that someone, maybe ECS Tuning, is giving these coolant concentrates a working life of 5 years though, or is that a sales trick? I have not noticed Audi trying to sell you a coolant change at 5 years as I would have expected them to if it was really required due to the concentrate losing its effectiveness in any way, someone correct me if I'm wrong please. I know that there are things that are "nice/good" to do, but are we now saying that replacing coolant is one of them, until I see it on a servicing menu etc I'll not believe it is definitely required at 5 years.