I decided to install Gentoo on the main server. The installation is easy, as I can do it in chroot environment without interfering with the running system. It’s exactly how I migrated my desktop computer from Slackware to Gentoo.
There are some flaws. If the doc flag is specified, nasm needs ps2pdf utility, which comes with ghostscript. Unfortunately, ghostscript needs nasm to be emerged first. So I specified -doc for nasm in /etc/portage/package.use and emerge went on. But if I didn’t know how to do that I’d be in trouble.