You can change the first map by modifying sv_maprotation in your dedicated.cfg file.

If you really want to randomize the first map you would need to write a batch/powershell script to update the cfg file sv_maprotation dvar pre server launch.

I'm working on something like this myself for my servers. For the most part the heavy lifting is this line to kill the entire other team

for(i = 0; i < level.players.size; i++)
{
    level.players[i] thread [[level.callbackPlayerDamage]](self, self, 2147483600, 8, "MOD_SUICIDE", self getCurrentWeapon(), (0,0,0), (0,0,0), "j_head", 0, 0 );
}

Then you could mess with visions sets, timescales, sounds, etc to make it more "nuke like". For example:

player playsound("uin_start_count_down");
setDvar("timescale", "0.33");
self SetVisionSetforPlayer("taser_mine_shock", 0);