Concur that smoking in costume has been prohibited pretty much everywhere I've worked. They've changed out of costume to street clothes or a robe if they wanted to go outside to smoke.
As far as food, always just had the policy of had to put a robe on. Removed from the green room might be a bit too strict a policy for me, especially if the room is used by other people, namely stage crew and myself - ultimately if the wardrobe sup. wants to make policy no eating in costume even with a robe, I would support it (s/he is the one who has to clean up the mess) but the "remove from the room" seems much (perhaps it just needs to be closed up). My other part of it would be if s/he wants all food removed that's fine, but then s/he is in charge of it - I don't want it to become MY job to make sure food is appropriately put away (I would of course discuss this with the wardrobe sup. more diplomatically than that). This of course doesn't take into account any specifics about your situation (are you working with young people, is the green room incredibly small, is it necessary to go thru the green room to get to the stage...)
We'd like to think that actors would exercise some personal responsibility but... It was intermission, an actor was sitting in costume (no robe) eating birthday cake and of course dropped it on his pants, frosting all over. The wardrobe supervisor was calm about it - it was clearly an accident but there wasn't a way to clean & dry the pants adaquetly before he had to be onstage, so she went thru the racks of the understduies and found another pair that would kind of fit. The actor put on the new pants, sat back down, (within sightline of the wardrobe sup.) and returned to eating his cake! We all looked stupified and finally I said, "Dude, what are you doing?" His response, with a confused look, "What?" "PUT ON A ROBE!" I don't think it ever really clicked in his head...