Thanks a lot for your replies, they were most helpful! However, I'm not 100% sure I understood what you said here:

Quote Originally Posted by Alexandr III. Biges
Yes, just don't forget to mark them as they may be a mercenaries and specify regions where they may be "built" only. The problem is, when these regions are not present near battle zone, than you have an option of raising mercenary magnetism of inns and/or decrease mercenary de-magnetism of castles and/or create new building with high mercenary magnetism and set them to specific areas as non-buildable (maybee even non-destructible, if it's possible).
If I have understood you correctly, this means that if I created a unit which was only available as a mercenary and limited it to be "built" in a certain province, it would only show up as a recruitable mercenary (in an Inn etc.) in the relevant province... So I could create a "Mercenary Desert Archer" unit and restrict it to only be recruitable as a mercenary in the same provinces as regular Desert Archers, and it would NOT show up in Inns in Italy or France.
Would this be correct? Sorry to be so bothersome.