In my design, I have the AEM author instance with its nodestore in MongoDB and its Datastore in NFS. The Publish instance has shares the NFS Datastore and has its own TarMK nodestore. There will be an offloading instance setup to offload some of the works of the Primary Author(like renditions generation etc). As I read from the Adobe docs, for this offloading instance, its Datastore can be the same as the one shared by the Primary Author and Publish instances.
However, what about the nodestore? Should this offloading instance connect to the same MongoDB database as the Primary Author or should it have its own TarMK nodestore? If it were to have its own TarMK nodestore will there be any compatibility issues when it is working with Primary Author which has its nodestore in MongoDB?
You should not add an AEM worker to your existing MongoMK cluster. The best approach for an offloading configuration is if you set TarMK instance for the worker and use share DataStore with your existing Author. It will also make sense to set a binary less replication to gain the best performance