This is not a bad thought, but it runs afoul of Apple's strange choices. If you view the "Custom" install of Xcode, there are five parts:
1. Essentials (1.75GB)
2. System tools (56.8MB)
3. UNIX Dev Support (563.7MB)
4. Documentation (Zero KB - sic! More seriously, it's a download at next boot, hence zero KB of stuff is installed immediately from the disc. Why they don't report the size of the later download, I don't know.)
5. Mac OS X 10.4 Support (Zero KB - again, sic!)
By default, 1-4 are selected and 5 is deselected. But here's where it gets interesting: you can only deselect 2-5 manually. 1 is a must. Care to guess where Xcode lives?
Here's how Apple describes "Essentials":
> Installs Xcode, Interface Builder, Instruments, Dashcode, Quartz Composer, GCC 4.0.1, GCC 4.2.1, llvm-gcc 4.2.1, GDB, and other developer tools. Also installs the Mac OS X 10.5 and Mac OS X 10.6 SDKs. All content is placed inside a location chosen by the user (default is /Developer on the boot volume).
I feel like John Belushi in the old "All I wanted was a pony, but nooooooooo" skit.
But you can re-direct the location for the installation of Essentials. It has "Other". Point it at an external drive (USB, whatever) and then wipe it. Make sure you install UNIX Dev Support - it will go onto the boot volume (and the gcc compilers come too.) And make sure to de-select "Documentation" if you don't like surprises later.