A very simple hub only needs robot probes / hub and distsrv to work.
As you add other things you might want to use like TOT /TTT or using AC2 then you need to add the following
ppm
nas
prediction_engine
baseline_engine.
Most of these probes do not need any configuration expect nas which you should enable replication on.
Please see the nas documentation on that
Also from the primary you will want to setup forwarding of at least licenses to the other distsrv from the one on the primary.
hope this helps.