Instant ownership requires map triggers.
The "Change House" action does the trick. <Player @ A> is the guy who starts at waypoint 0, <Player @ B> starts at 1, and so on.
The best practice of this is to not make it "instant". Use a time delay of 5 seconds, because skirmish AI players get confused when their first building is not their conyard. Else they often build shipyards near the free buildings instead, or build base defenses where the free building was, once it gets destroyed.
Further on the 'good practice' idea - the free buildings that are on the map should be Special (JP) House. Then you use a trigger which fires 1 second after the 'Change House' triggers do, and that trigger should seek to destroy everything on the map belonging to JP House. In this way you can clean up buildings that go unassigned in the event some player start points are vacant.
How do I give the american side the grand cannon weapon
