Since they are already together, I would not bother trying to quarantine them.
I would not give them a bath, unless there is something specific that you need to wash off. Baths are stressful for chickens, and do little or no good in most cases.
I would pick up each chicken (new and old) and look it over thoroughly. Also watch for any problems with droppings, or any other symptoms, over the next few weeks.
If you find any specific problems, treat the problems you find (lice, mites, worms, diseases.)
But if the chickens look healthy and act healthy, just let them live like normal chickens, and keep watching. If you try to treat everything they "might possibly" have, you might end up with sick or dead chickens just from the side effects of treatment they did not need!
No, they should NOT see and hear each other during quarantine.
The whole point of quarantine is to keep ALL germs, including airborn ones, separate. If they can hear or see each other, they can spread germs too.
If someone is going to quarantine chickens, they should be COMPLETELY separate. The look-no-touch step of introduction happens AFTER quarantine is over.
https://www.backyardchickens.com/ar...nderestimated-part-of-raising-chickens.67097/
In OP's case, since the chickens have already been put in the same pen, I do not think quarantine makes any sense.
In many other cases, quarantine is not really possible. When quarantine is not possible anyway, trying it just adds stress for the people and the chickens. Housing chickens in adjacaent pens is a fine way to start introducing them to each other, but it is not quarantine, and should not be confused with quarantine.