[Edi]

As far as what determines the paths of your mages, the magic paths are an intrinsic part of the unit itself, so a mage may have certain paths fixed and some randomly determined (randoms can only be less than 100% sure, so not all of your mages always get them all).

Units that have been intended for a specific nation ahve been designed with that nation in mind, so for example the magic paths of mages reflect that.