Sme 7.0 beta has transparent proxy, so if you configure the clients the one way or the other, the trafic will pass trough the proxy (if it is turned on.)
Im not sure if this also the case for 6.5 I think you can take a look via the program "top" or in the Squid log file to find it out if this apply for the 6.5 as well.
If it is not a transparent proxy there, I think there is then no standard way to force the clients to use the tcp 3128 client connection. Of cource it is possible to manupulate the firewall so there is only a tcp 3128 way out, but I believe that there is no "standard" or "control panel" solution for that.
Arne.