I want to setup a firewall for a network of 200 devices. Can anyone please guide me which GNU/Linux distribution to use ?
Depends on your knowledge. Most commercial firewalls which use a unix as a background would use bsd/freebsd.
Me, I like slackware, but it depends what you want and how much you know. Redhat of course comes with support (because you have to pay for it and its against all their stuff to use it without support.. sigh) but if you dont know much about linux then this maybe a better option because they can help you.