I don't know alot about coughing in cats as mine have never done anything like you're describing, and we rarely see ones doing that at the hospital, most of those are sneezing not coughing.

As far as watching to be sure 6 cats are eating, I'm having to assume you free feed them? I have 7 cats (used to be 8) and I feed them several times a day, keep everyone in their own food bowls for 10-15 minutes, then pick up the food bowls so I always know who's eating and who isn't. If I have one I'm monitoring for bathroom habits etc, then I shut that one in a room by themselves.