Hmm…installing a different GCC in macports looks like it just dumped it all in its own subdirectory. I'm not so interested in cross compilers, just different versions of various compilers.
I don't know why GCC would have problems with it unless you used the same $PREFIX for all of them. It's not like GCC would know to look anywhere other than its installation tree and possibly the system. Usually I use $HOME/misc/root/$name-$version as the prefix so that removing one is an rm -rf without having to rely on whatever (usually broken) uninstall mechanisms the project uses. LLVM is certainly happy being installed to $HOME/misc/root/llvm-3.2svn without being confused by the system version.