Not sure whether there is a way to have Squid limit the available bandwidth for downloads during certain times of the day.
As for blocking MSN, it uses multiple different ports to access servers - I tried blocking certain ports and ranges, but the most effective is to ammend your iptables rules to drop all outgoing TCP connections except, say 21, 25, 80, 443 which would still allow your main internet protocols to work, but drop all others. this was pretty effective, but there are quite a few web-based messenger services such as www.e-messenger.net
and so on, so then you start blocking sites through squid,squidguard or whatver and it keeps going on and on! hope this helps a little though!