Material export creation. Doing some texture baking for a bunch of geometry and then exporting as FBX files.
For creating simplest materials containing only Normal and AO maps, should the Principled Shader be used?

Not sure how the AO maps to this node.

That is correct! Principledshader has the best support throughout various tools (Baking, FBX, etc). That would be the one to use.
As for AO, that is a request I see often (and have requested myself). Could you please send in an RFE for this?