As far as I know, the game does not indicate that there is a specific prohibition against mages marrying. That said, there are no known married mages, and the requirements of the job seem to suggest that it would be difficult for a mage to have a family. It is also worth noting that most people in Cythera are still wary of the mages, trusting them because Alaric wishes them to. It is possible that marriage is permitted but is almost unheard of.
My guess is that the isolation would include families, although there seems to be no prohibition against visiting a freemage or a freemage passing through a town. Timon actively joins your group which would seem to violate the requirements, but Lindus might have granted him leeway given the circumstances.
Given the relatively low number of judges, it is possible that every mage has to fill the position at some point or other but that it can be deferred until after one has served as a freemage. Alternatively, there may be more judges than seen in the game or, as you suggested, mages might meet this requirement through some period of a civic service.