Page 49, Militia, “Militia can only move within a Major Power’s Home Country.”
Militia cannot use strategic movement if they aren’t in home country. A great example is the Tannu Tuva militia. Tannu Tuva is not home country for the USSR, so you cannot move the militia. Because militia can only move “within” home country territories, I don’t see how one could move militia by transport. If you moved militia via strategic rail movement, it would need to start in home country, end in home country, and trace a path through only home country territories.
I don’t know of a specific rule forbidding Spanish nationalist or republican militia from moving and attacking in Spain, but I think there should be such a rule due to the precedent of Chinese CCP and KMT militia. Chinese militia cannot move within China until after China evolves to a major power. This would suggest to me that Spanish militia cannot move and attack in Spain. You could make an argument that since there is no defined list of Spanish home country territories, then the Spanish militia is not allowed to move. I guess that’s where I’d come down on that particular issue.
I have found militia you gain from aligned minors to basically be permanent garrisons. You cannot move them. One option is to upgrade them to infantry (Page 47), but remember this requires a supply path. If the USSR wants to make use of the militia in Tannu Tuva, it will require that a rail be built into Tannu Tuva. Since that territory is mountainous, such a rail would cost 4 IPP, as much as 2 militia! So the only thing that makes sense is to leave the militia in Tannu Tuva as a permanent garrison unit.