I need to top up the coolant bottle. The manual recommends G12 Plus (TL774 F). I believe the one I have at home meets the requirement of TL774 D. Can they mix?

No. G12 or G12+ only.

There are two main types of coolant IIRC - and from what I remember (having done this wrong, many years ago) if you mix them you get a nice slurry which will clog up the radiator at least and will take some doing to get rid of again.

The good news is that G12(+) is not too expensive, and if you are in the UK, you can also usually get it at Halfords for example in case there is no other dealer available.

If it is a little low you can put a little water in to top it up, but keep an eye on it. If the level keeps going down, you want to really find out why it is happening.

I think TL774D is original G12, whereas TL774F is newer G12+ , the difference is that G12+ will mix safely with the old G11, whereas G12 will not. All G12 is pinky purple colour. G11 is greeny blue.

I find that coolant levels always quickly settles near the minimum mark. If I add more, it just mysteriously disappears in a couple of days and its back to the minimum mark. So I just leave it there and dont try to fight it. Happens with every VAG group car I've owned.(5)