It seems that Unity and xfce are quite OK, they don't really bite each other. I have them both installed, although in practice I only use xfce. Other combinations may be less fortunate.
Something that could be used as a recovery point for installed packages is this:
where your-recovery-point.asc will be a file in your home directory (or wherever you want to store it). You may want to encode the date in the file name. The only catch is that dependencies may have changed, which has to be handled correctly.
#To create the "recovery point":
dpkg --get-selections >your-recovery-point.asc
#to restore to a recovery point
sudo dpkg --clear-selections
sudo dpkg --set-selections <your-recovery-point.asc
sudo apt-get dselect-upgrade