Can I use SELinux with amd64 no-multilib?
Yes, just use the hardened/linux/amd64/no-multilib/selinux profile and you're all set.
Can I use SELinux with systemd?
Yes, in 'theory' it is possible to use systemd with SELinux. Anyway, on Gentoo it’s still in an unstable and not well tested state and therefore masked via the SELinux profiles. If you want to try it anyway, you need to unmask systemd (package and package.use) by yourself.
In every case take a look at bug #528674.