I don't know your set up, and I don't know squid, and I am not sure that it's as configurable as you need but you could always try 'Trickle'. If you have the a ability you can enforce a policy whereby users use their internet accessing programs through Trickle.
It can be used like this (perhaps through a shell script):
trickle -d 20 -u 5 wget -c http://mydownloads.com/bigfile.zip
This will start 'wget' with the specified options and enforce a maximum download speed of 20kbps and an upload speed of 5kbps while it runs. Of course it can be any other application, it doesn't have to be wget.
[Update]: Have a read of this page: Squid Proxy Server Tutorial Specifically check out point 4. It seems to detail exactly what you're after.