I don't know that switchtower is your best option here. Why not simply
set up the main computer as an NFS or FTP server with the movies on it?
You could have the other computers load a file every minute or so that
would contain instructions to play a certain movie file at a certain
time. This is the simplest way I can think of and would also guarantee
that they play it together if their clocks are synchronised (the main
computer could be set up for NTP as well).