"My sons, you must wear shoes."
Translation:Mes fils, vous devez porter des chaussures.
You're talking about the English point of view. Indeed in English if you use "must", It can be related to a rule applied to everyone. But in French, it's "devoir" both for "have to" and "must".
In a French point of view, "Mes fils, vous devez porter des chaussures." can perfectly apply only to the sons, and not to other people.
Well, because "porters" is not a French word. The correct word is "porter".
And for "soulier", it's an old word which is now most of the time replaced by "chaussure". But technically it's correct to use "soulier" if you don't have a context, because it's still used in literature and stuff like that.
Your previous response regarding an "Il faut" construction doesn't actually address this, since you describe "Il faut" being used regarding a rule that applies to everyone. "Il fault que vouz..." is addressed to one person/the group of people addressed. While it could be an instance of a general rule being applied to an individual, wouldn't it also be usable to tell an individual/group what they must do? For example, "il faut que tu partes".