Alright, putting all the answers together I think my initial question can be answered as follows:
Checkmk will be compatible between version 1.6 and 2.0. However it is strongly recommended to not stay in that state. It should merely exist during the upgrade process.
Thanks @andreas-doehler for sharing your experience and @martin.hirschvogel for the documentation!
I will update this after I performed the upgrade in my environment. Anyone able to share their experiences feel free! Maybe someone else is looking for this and can gain confidence from this post.