Hi @Ciara_Sabud, welcome to the forum!
As of right now, there isn’t a way to create “global rules.” You were on the right track by adding the rule to your Master Project. It’s much less work to add a rule to all your projects that multihomes tasks into that project, rather than creating separate rules in every project to apply those preferences.
Unfortunately it may become more complex if this is also needed for subtasks and/or recurring tasks.