One suggestion would be to install Internet Management sytem such as Websense (expensive, for bandwith limitations you need a separate license - more expensive), or you can try delay pools configuration in squid - read
http://www.tldp.org/HOWTO/Bandwidth-...WTO/intro.html
pay close attention to the delay pools configuration - they use only one global network, I believe if you define multiple acls for your network segments you can create separate delay pools for them, the only drawback is how to configure it for percentage of the available bandwidth, search on google for "configuring squid" or similar, I am pretty sure you will stump on something usefull. Good luck.