Hum
You can't define specifically in a map what nations are/aren't allowed, you can only set the max nr of "land" (#landplayers X)and "sea" nations (#seaplayers Y) respectively.
What you can do is to use #specstart to define the starting province of the nations you want and "no start" flag to all provinces, so that all other nations will generate an error when starting the game
Drawback is that starting provinces will always be the same...