you can add more interfaces on the outside, and have a different subnet/vlan for each interface.
You can also use a single interface in trunk mode.
No need for secondary ip addresses.
However, this kind of config is more complex since traffic can come in on 1 interface and go out on another one.
It is better to put a single interface between CSS and servers vlan and another single interface between css and an external router which will have a connection to all your vlans if needed.
Let a router do the routing and the css do the loadbalancing.
Gilles.