What are the rules for calling "at ease"?

The general rule of thumb for this custom and courtesy is that: When the senior ranking NCO enters the room to conduct an inspection and exits the room after the inspection, At Ease is called. So yes, even if the E5 is staying, you call At Ease when the E6 inspecting NCO departs. Again, this is for inspections. For further use of At Ease in regards to customs and courtesies, you wouldn't necessarily use it for every single NCO within the unit. What I mean by this is: This particular custom and courtesy is reserved for the senior ranking NCO of the element...Squad Leader, Platoon Sergeant, 1SG, and CSM. So, if your squad is hanging out in the Day Room and the Squad Leader walks in, then At Ease should be called. If the entire platoon (minus the platoon leader) is in the Day Room and the Platoon Sergeant walks in, then At Ease should be called. And so on and so on.

I believe you would only call it again if the room at returned to a lower state such as if the SGT had said "rest" or "as you were". Otherwise the room is all ready "at ease" when the SSG enters. In the case of the SSG leaving and SGT staying behind, it most instances the room could stay at it's current state or the SGT could take the room to a lower state if they desired. I would equate this situation to a platoon that was at the position of attention when the PLT leader posts and replaces the PSG. There is no need to call the formation to attention since they are already there. The again, it has been close to 25 years since I did any drill and ceremony.