As far as I know there is no "fixed date" for the origins of Slava.
What I do know is that it was not directly Cyril or Methodios who started it, but rather their disciples, or the disciples of their disciples, who were the real Christianizers of the Serbs.
The Serbs were probobly amongst the last to be "targeted" in the mission work of Cyril and Methodios. So, I would say mid 11th century (around then).
If a Serbian couple gets married then good old male dominance takes over. The male's Slava takes pre-eminence over the female's Slava. Patriarchal society and all. The girl can still celebrate her Slava, but its not as big of a deal as the guy's Slava.
Also the Slava itself is hereditary. So if you have a son, the son will take on the family Slava after the father passes away.
If you marry a Serbian person then you usually take their Slava. For example, if a guy marries a Serbian girl, then he will begin to celebrate her family's Slava, even though she is a girl. This is the usual practice.
The only time you get variances is if a couple or a person were to CONVERT into the Serbian church (which is rare
). If this happens, then they will probobly be given the option to chose their own Slava and protector saint of their family.
I hope this answers your questions!