I have never used gentoo. Does gentoo patch packages as much as Debian? Or is their build process sophisticated to allow parallel install of multiple versions of say postgresql?
The build process is also sophisticated enough to be able to 'slot' potentially-incompatible versions of the same package so that they can be installed simultaneously. From my recent snapshot of the package repo I can install 5 different simultaneous major versions of postgres, with a variety of independent build options.