I had some issues getting Squid to work with both SRX and EX based forwarding rules. All worked perfectly with a Blue Coat ProxySG.
For Squid, I needed to add some stuff to iptables -
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3128
For anyone interested, theres a full blog entry on our company website for doing it all with EX or SRX -
Transparent Redirect with JunOS
I used the WXC type redirect to make it work with the Blue Coat ProxySG.