If you change trains after midnight, then you need a new travel day, regardless of the type of train.

The connection you mention would not be a direct train. Arrival at 6:18 sounds like arriving with the night train from Munich but coming from Zurich you'd have to make a few changes in the middle of the night. It wouldn't be my preference :)

If you say what the travel date is, then people can make suggestions.

Use bahn.de for planning and click on details to see what trains are listed and how much time in between. AND learn those age-old pass-users tricks, though they have changed since in the old past it was just reverse with the rule for using overniters. As you have learnt, you can BOARD any train on a chosen travelday, even if you alight after 0.01. So you can go to M=MÜnchen in DE in the late afternoon, and then board the overnite southbound till destination (check yourself if Milano is among them). These roundabout routings will never show up in any planner. That way you do not have to spend hours in the midst of night on some cold&windy platform among the homeless people. Though for some this also forms quite an enriching experience.