I'm fairly sure cauliflower leaves are allowed. My rabbit's an indoor 'free-range' one so it becomes obvious fairly quickly if she's eaten something she shouldn't. She has cauli leaves, broccoli, carrots, dark green cabbage and sprouts. She is occasionally allowed a very small amount of celery leaves (in small doses she's okay with them). She only gets a small portion of green stuff at bedtime (one carrot and a bit of something green). The rest of the day she has her hay and normal food.
We tried her with carrot leaves but these do not agree with her even though she loves them.
spinach contains high levels of oxalates/oxalic acid which can give piggies kidney stones, apparently. Mine get the occasional leaf as a treat (they love it!) but not regularly. You're not supposed to give them lettuce either, so one guru book told me - so again they only get an occasional treat of it.
I've never heard of cauliflower leaves being a problem and have always given them to my pigs with no obvious problem, but not every day as I don't have them available every day! ANd they do rather love them too...