Hi,
We use the SafeSquid Composite 20 edition (free) as our content filtering proxy and application layer firewall. It has a feature 'Limits' that can be used to throttle QoS based on user, group, website being visited, mime type, size of content being downloaded, time, etc.
You can also define other parameters like Download transfer limit, Upload transfer limit and no. of Request allowed.
It also has a feature 'Multi Proxy', that can be used to control multiple SafeSquid proxies, installed in the same or remote locations. One SafeSquid server is configured as Master, and the remaining as Slave. All slave servers pick up rules from the Master. SO now you need to configure only the Master Server, while the slaves follow. You can also define specific rules for specific slaves.
Of course, it also has many other features like url filter, url blacklist, mime filter, cookie filter, header filter, built-in clients for a number of open source (ClamAV) and commercial AVs, universal ICAP client to talk to any ICAP server, content caching, ICP, CARP, request forwarding, url redirecting, document rewrite to block scripts like activex, java, appalets, etc., PAM to talk to external authenticating servers, etc. etc. etc.
If you would like to try it, you can download the full features free version that can be happily used in a 20-30 user network, from
http://downloads.safesquid.net/free/...20-free.tar.gz
You need to register on the forum at
http://www.safesquid.com/forum and use your ID to access the downloads site.
Best Regards