Yes and yes. I have 10 of them in 2 of the rack mount accessories. The rack mounts can hold 6 as you note but come with a couple of blank panels you can use if you don’t fill them up. Very quiet also. They do have a fan but it is whisper quiet.
You should be aware that there can be a group sync issue (where grouped zones aren’t playing in sync) with these (and all blueos devices) and roon. I see this occasionally.
There is a thread about it here: Third speaker out of sync
Of course not an issue if you don’t intend to group zones. The workaround I use if needed is to group them in the blueos app that doesn’t have this issue and then play them in roon (roon only then sees this as one zone).
Bluesound has said they are working to fix but have given no date.
Other than that, they are sweet units.