Your question seems a little foggy, so I'm going to answer it as best as I can (though this may not be the exact answer to the question).
Is UnixA also routing traffic between its networks? Is routerA multihomed? Do you want all traffic on the 192.168.100.x subnet to go through RouterA? If so, then make that the default gateway for all those boxes on that subnet.I need to get the traffic on UnixA eth1 to route to 192.168.100.101.
Traffic on UnixA--as long as the traffic is local to the 192.168.100.x subnet--should be automatically sent out eth1, directly to its destination. If all your boxes are on the same subnet (192.168.100.130, 192.168.100.100, 192.168.100.101) then your routing tables will not be consulted at all, unless the destination IPs lies outside of that subnet. *
Subnets *rely on ARP brodcasts for local address location. Only when ARP broadcasts fail to return a MAC address for a given IP will a computer then send packets to its default gateway. If you want packets for destinations other then the 192.168.100.x subnet to go through 192.168.100.101, then you will need to make that ip UnixA's default gateway.
Disable the external interface (eth0) if possible or disable ip forwarding (if its enabled). Make 192.168.100.101 the firewall's default gateway.No I need to add a route on the firewall so it is not forwarded out eth0 like all the other 192s but instead is sent to the router.
I hope this helps a little bit. Like I said, it is a strange setup. Perhaps if you told us what you to do in the end we can be of more help.