I would not think that we can mix 2018 and 2019 cars. Simply because BoP changes happen between seasons. Does also not make any sense to mix the cars of both seasons in terms recreating Blancpain GT. I understand the notion that you want to, however.
With a 2019 line-up the Nissan will not be usable for example.
I was wondering if we get a second championship route with the 2019 cars without affecting an existing 2018 championship in progress or finished.
Looking forward to exploring the new content, that is for sure!