a tcz extension emulates what in slax is done via aufs by symlinking.
This embeds it in the fhs, allowing it to be a dependency of other tczs.
A scm skips the symlinking step, because less importance is paid to the dependency mechanism.
Ok, it sounds reasonable that two different types of extensions have been conceived, given this rationale.
However, I am in the situation of needing a interdependency mechanism between tcz and scm.
In concrete: I have created a minimal texlive 2012 extension, which I need to be a tcz because of integration with the fhs and because I sometime need to tce-load -ci to do modifications.
I modularized the additional stuff from texlive into scms.
That is because such stuff is made from a myriad of small files, and even just symlinking all of them wastes space (checked that using du /usr/local/texlive).
(by the way, this illustrates another advantage of scm over tcz)
Hence: is it possible to have a tcz as a dependency of a scm?
Currently, it seems it's not...