Can't you enable the compositing features in metacity in CLI with
gconftool-2 -s '/apps/metacity/general/compositing_manager' --type bool true
and
Section "Extensions" Option "Composite" EndSection
in the /etc/X11/xorg.conf file as documented on the AiglxOnFedora Wiki page?
No, you can't. As I said: "metacity" is built -without- compositor functionality. Setting the "gconf" does not change anything.
BTW, what will happen when both compiz and metacity with composite features enabled are installed on a system?
You will simply have 2 compositing window managers between which you switch by means of activating "desktop effects". Options that you can checkmark of course only apply to "compiz". "metacity" compositing effects are not supported in the user interface. Btw, your questions had been discussed exhaustively earlier this month in thread "Wither bling?" at:
https://www.redhat.com/archives/fedora-devel-list/2006-August/msg00362.html