[gentoo-user] Installing Gentoo from another Linux distro

[Grant]

I'm getting ready for a remote Gentoo install.  I'd like to avoid any
sort of CD/USB/floppy usage.  I can select a variety of different
distros to be preinstalled on the system.  Can I end up with the same
Gentoo system installing from another distro as I can from a LiveCD?
I've read over:

http://www.gentoo.org/doc/en/altinstall.xml#doc_chap5

and it sounds like I may not be able to end up with one big root
partition like this:

Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1           5       40131   83  Linux
/dev/sda2               6          68      506047+  82  Linux swap / Solaris
/dev/sda3              69       19457   155742142+  83  Linux

If the preinstalled distro has a partition table like this, I won't be
able to make a separate partition for Gentoo.  If the preinstalled
distro has separate large partitions, I will be able to install Gentoo
on one of the partitions, but I won't ever be able to set up a
partition table like the above with one big root partition.

Is that correct or do I have this all wrong?

- Grant