Juniper SSG5 And Public IPs

we have this Juniper SSG5 firewall, our very first Juniper and wanted to use it.

While I am able to use it in NAT mode, I have been unsuccessful to use it in route mode.

We have Public IPs from the same segment and I wanted to use it with the firewall but it appears that I can't assign IPs for each port if the IP is from the same segment.

Does it mean that we can't use it other than in NAT mode?

What we want to accomplish is to have one of IPs to act as gateway and filter or route in/out traffic to/from our other IPs.