Definitely not 6". Use 4" or even 3". I gather they all have to go in the 2' space around the tray?
yes. they are all in the 2' tray. agree 4" looks good,
now my next challenge is the 4" IC cans are still too bulky and it doesn't fit on one side of the tray as there is drain pipe occupying half of the 2' tray width.. so if I use the non-IC cans (smaller), I'll not be able to put the fiberglass insulation - how would I then fill the cavity then (otherwise, would there be resonance due to the hollow tray?)