Remember that once you move actors to a new level, or create a new level from selected actors, they are no longer in the persistent level.
So yes, if you delete a streaming level form your Levels Details list will remove all those objects from your game. You can simulate this, but clicking the eyeball and watching them turn off. You can do it by “delete” the level:
RMB -> REMOVE SELECTED (see it’s not delete because the level doesn’t get deleted from your disk. If you want to then bring it back in go to Levels above dropdown (in the Levels Detail) and :
LMB -> ADD EXISTING
Yes you can have your team work on levels separately, but I would tend to do want to be careful about this. It’s really easy to accidentally work on the wrong Level, I do it constantly. You can Lock the level that someone else is working on. I think though I would take the tack of “publishing” levels to an area that they get copied to when the person working on it feels they have made significant progress.
So if you set up your Levels, and there are additional streaming levels like, Blue_Hallway, Red_Hallway, Cathedral_Main etc, I’d lock those in your game level that brings all that together.
Have your people work on checked out versions of those levels, so Blue_Hallway_v2 (version 2). At the end of week or whenever they “publish” they copy the Blue_Hallway_v2 into the folder that has Blue_Hallway and overwrite it. It must exist already having been made in UE4 for this to work. UE4 doesn’t just see things copied into folders willy-nilly.
Then the person working on Blue_Hallway_v2 should save as Blue_Hallway_v3 and work on that not v2 any longer. Keep version backups to keep yourselves from having a major accident. If someone overwrites Blue_Hallway_v2 by accident and you do daily updates, then you have only lost a days work.
There are other techniques for multiple developers to work together, but I haven’t investigated the UE4 techniques for this, because I’m a solo developer. However, I do work with many levels in my game, and often I work in not the Persistent level to get at a particular level, like Blue_Hallway, but I just load up Blue_Hallway remembering that if i add lights or stream in my global lights then i have to remove them when I save out.
I take any lighting that should be global to my game, like sky, the main directional light, and fog, and put those in a GLOBAL_LIGHTING level. That way if i am working on Blue_Hallway, all I have to do is stream in that GLOBAL_LIGHTING while I work.
Be mindful of lighting though. If people are sharing lighting things can get quite tricky. It may be better to have separate lighting, but an individual can do the main lighting, save it as a GLOBAL_LIGHTING file, and get that to the team, so they can copy those lights and use them as a starting point for their scene. In vfx we called this pre-lighting, or Look Dev, which is a much cooler title.
Look Dev/Pre-lighters decide the look and how things may work, and get that out to the team so people have a place to start by their lead, and don’t waste time doing the same look development.