Your problem probably has nothing to do with how you built the iso.
Many contributed packages require certain services such as http to be running before they can properly install. twiggi, for example, wants to be installed on a system that has all services already running before it is activated.
The work around is to bundle up your own rpm of packages and have that installed into a subdirectory on the server (for example /opt). Once that is done, you can run a script also installed by your new rpm in rc.local that includes logic that will do a one time install those apps on the first complete boot.
So, roll up your required apps (twiggi, etc.) into an rpm called mynewstuff.rpm. Drop that into the iso and edit comps accordingly.
Once you can prove that it is installing the new rpm, then start the process of writing your install script.
Hope this helps....
-jeff