I think my original post didn't make clear that I know where the servers need to sit in terms of subnets (inside/outside), I just didn't know if enabling the firewall caused issues for normal routing (from the T1/frame relay interface) on the public interface.
Let me confirm, for example:
7102dl config (frame relay, but just showing the net interfaces):
eth 0/1: 192.168.0.1 (Private)
eth 0/2: xxx.xxx.xxx.1 (/27 Public IP Block)
secondary eth 0/2: xxx.xxx.xxx.2
I know I can sit a server with ip 192.168.0.2 on the private network (gateway 192.168.0.1) and one-to-one NAT xxx.xxx.xxx.2 to 192.168.0.2. That shouldn't be a problem.
The part I wasn't 100% sure on was, once the firewall is enabled, can I continue to have servers assigned with the remaining public IPs talking to eth 0/2? I'm thinking this will work fine, since that interface is still visible, I just didn't know if I'd run into problems enabling the firewall to NAT a few of those public IP's.