In honesty, I like Gentoo. We switched our entire network to SuSE based on 2 things. We wanted to keep KDE, and we needed something that Scalix and Oracle would support.
If Scalix was to support a "community" distro, I'd personally recommend Kubuntu/Ubuntu over Gentoo or Debian. It has a more or less consistent release system. And it has some financial backing, and a number of volunteers behind it. Personally, and it makes me sad to say it, I suspect it'll kill off Gentoo and Debian in the next two years. Which is fairly ironic, since it's Debian based.
But I've never been good with a crystal ball.
Kev