Yeah both runit and sysvinit are supported, but packages are no longer required to include sysvinit scripts, so there’s no guarantee that all software will work. Most have kept their sysvinit script though.

The main issue will be that systemd does a lot of stuff, so you’d have to install replacements for everything else it does - like a syslog daemon for logging, ntp client for clock syncing, DNS resolver, etc.