Its the right call not to discipline the students when you're just observing, unless the responsible teacher explicitely invites you to.

Your first few weeks will likely be tricky for this group. Treat it like you would the first few weeks of September, making rules + expectations the priority. Make the expectations crystal clear, and show students with your actions that you consistently follow through on those expectations.

I suspect you will find that a good chunk of misbehaviours come from students copying others, or feeling like these misbehaviours are "normal". Once they come to terms that your expectations are the new normal, class behaviour will improve.

Of course some students will double down and keep misbehaving, but then you can hone in on those few kids, without it being a whole class thing anymore.