You got pretty much everything right.

The beginning of the sentence means (roughly translated): “If only one of them get in our things”. So the “Que” stands for somewhat close to “If only” ou “Rien que” in french as Luke said in the comments.

P.S. You got a wrong conjugaison of the first verb in your example. It’s either “Tu devrais” (which makes sense with the remaining pronouns) or “Vous devriez”