Actually, it's just common sense, considering that only players from the tiny Serbian ethnic community inside Kosovo declare for Serbia, the rest declare for anybody but Serbia, but mostly Albania.
Kosovo stuff
Kosovo status
It is understood that once Kosovo is a full FIFA member, any eligible players will have a window of opportunity in which to switch allegiances from their current teams to Kosovo.
Montenegro stuff
Montenegro wiki page
the chapter
Current Squad
In international football, players can normally only play for one national team once they play in all or part of any match recognised as a full international by FIFA. However, an exception is made in cases where one or more newly-independent states are created out of a former state. Based on current FIFA rules, a player will be eligible to play for Montenegro, even if he had previously represented Serbia and Montenegro or any other country, if at least one of the following statements applies:[9]
- He was born in Montenegro.
- At least one of his parents and/or at least one of his grandparents was born in Montenegro.
- He has lived in Montenegro continuously for any two-year period.
Due to mixed ancestries, it is likely that a high percentage of the players eligible to play for Montenegro will also remain eligible to play for Serbia, and vice versa. However, once they have played for either Serbia or Montenegro in any competitive fixture, they are no longer eligible to play for any other nation.
There is a linked reference to a FIFA pdf document on the status transfer of players, maybe there is something more in that document.
But I'm satisfied enough not to pursue it any further :)