I got the BMW M6 GT3 already, if I would buy the ADAC pack with the containing GT3 cars will I then be able to drive against those with my BMW? And the other way around? Or do I need to buy the GTR3 pack for that?
In general, if you own one car of any class you can drive against all other cars in that class. Competing with cars from different classes (ADAC GTM and GT3 in your scenario) is only possible in multiplayer atm, but here the same rule applies. The only determining factor is the server configuration, some servers are configured to only allow one particular class, other servers mix classes. If a server is set up to run both GT3 and ADAC then you will be able to drive against all the cars from both classes as long as you own one car in either of the two classes. But afaik there are quite a number of servers that mix ADAC and the GT3 class. If a server is set up to run all available classes (there's a Nordschleife Tourist server I know of, but there's certainly more) then you can drive against all other cars from all other classes as long as you own one single car.