Limiting bandwidth dynamically in Squid
You could use
acl aclname rep_mime_type mime-type
to block certain mime-types from downloading (eg iso's)
and
reply_body_max_size
to set a ceiling on the biggest files that can be downloaded
you could also use free-sa to analyze your logs to see where the bandwidth is going
http://free-sa.sourceforge.net/
EDIT: Actually squid delay pools will do EXACTLY what you need.
read this article: http://www.howtoforge.com/squid-delay-pools-bandwidth-management