All configs you want for new users goes in /etc/skel which contents are being copied for new users.
Beware some config packages in synaptic (like econfig, econfig-light) might overwirte the configs in /etc/skel when they're updated.
To make a remaster with a guest user out of existing install:
1- Change root password to root
2- Remove the existing user(s) leaving their home in place
3- Create a guest user with uid 501 with password guest
4- Set up the config for guest and root and copy the configs from the guest to /etc/skel for new users.
5- When running mylivecd exclude all custom user directories except /home/guest
Some tips on cleaning up:
1- Remove all files from /var/log recursive except prcsys.log
In a terminal cd to /var/log, move prcsys.log to another directory and run this command:
find . -type f -exec rm {} \;
Then put prcsys.log back in /var/log/
2- Remove all .rpmnew, .rpmold and .rpmsave files
cd to /
Then use the following command
find . -name "*.?" -exec rm {} \;
Replace the ? with the extension you want to get rid of
(example: find . -name "*.rpmold" -exec rm {} \; )
3- Remove .rpm files from /var/cache/apt/archives
4- Run bleachbit as root and guest to clean out both accounts
5- From /root/ and the guest's dir also remove .thumbnails and other configs you don't need (like .macromedia, stuff in .cache, unneeded configs in .config)
6- If you want synaptic to be empty like on an original iso remove .synaptic from /root and bot cache files from /var/cache/apt