Are there rules to follow?
Are there infomations available somewhere?
It depends, what the daemon does and what it depends upon.
I don't think there are any written rules more just common sense.
What daemon are you trying to add?
The first three that come to my mind are chrony, fcron and connman.
The sysinit and boot runlevels are populated and managed by the default openrc installation. Most user-added daemons (like the ones you listed) should go to 'default', unless you really know what you're doing.