hi,
do you have other interfaces in your ssg 550m? uPIM?
i would recommend to have additional pims so you can assigned each ports for isps as well as your required zones for the vlans. (to get things easier, since default port is only 100gb copper)
VLAN1 by default is used for screenos transparent mode deployment.
Once you assigned individual ports on respective isps, you can have them active/active mode by utilizing multiple virtual routers and source-routing (trust-vr to isp1-vr, isp2-vr, isp3-vr).
//
dwayne