Unless you have a completely closed group I would suggest that everyone get tested every 6 months. When you get tested get tested for everything, after all it's better to know than not to know.
The only person who's behaviours you can control are your own. If you become uncomfortable with a partner or a metamour's sexual practices you can insist on using barriers with that partner. I think it is unfair to make a rule that someone would have to break up with a partner due to a previously existing rule. I think a better guideline would be that everyone is open and honest as to what is going on and that no one within the group should take offence if any one of you chose to ask that barriers be used again.
Everything will be ok in the end. If it's not ok it's not the end.